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Full text of "Thin film materials through the interfacial assembly of inorganic networks"

THIN FILM MATERIALS THROUGH THE INTERFACIAL ASSEMBLY OF 

INORGANIC NETWORKS 



By 
JEFFREY THOMAS CULP 



A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL 

OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT 

OF THE REQUIREMENTS FOR THE DEGREE OF 

DOCTOR OF PHILOSOPHY 

UNIVERSITY OF FLORIDA 

2002 



For Garrett 









ACKNOWLEDGMENTS 

It has been said that no man is an island unto himself, and nowhere is this truer 
than in the scientific community. Experiments are proposed, results are discussed, 
conclusions are debated, and out of skepticism, truths are extracted and new ideas are 
born. Indeed, the life's blood of science is collaboration and I have benefited greatly 
from the countless discussions with fellow colleagues throughout the course of this work. 
First and foremost, I would like to thank Professor Mark Meisel and Ju-Hyun Park in the 
Physics Department at the University of Florida for performing all of the magnetics 
measurements presented in this dissertation. Equally as beneficial were the many 
discussions the data provoked. I thank you both for your hard work, your knowledge that 
you shared with me, and above all for providing a working relationship that was just as 
rewarding personally as it was professionally. 

The data collected at the Advanced Photon Source (APS) were essential to the 
work described in this dissertation and I would like to acknowledge all those who assisted 
in the experiments. I would like to thank Professor Randy Duran for getting the 
University of Florida involved in the Materials Research Collaborative Access Team 
(MRCAT) at the APS and for our fruitful discussions. The beamline support staff at the 
MRCAT were a valuable asset and I would like to especially thank Nadia Leyorovska, 
Holger Tostmann, and William Lavender for their assistance. I also thank Professor 
Pulak Dutta and his group at Northwestern University for their aid in getting our 
Langmuir diffraction experiments off the drawing board. To my fellow researchers from 

iii 



the Duran group at UF, I want to say thanks for making the long hours and hard work 
bearable with your assistance and your wit. Lastly, I want to thank Guyanga 
Weerasekera and Dr. Mark Davidson who were my left and right hands at the beamline. 
I do not know how I could have done it without them. 

My day-to-day research was made all the more enjoyable by my friends and co- 
workers in the Talham group. I would especially like to thank Gail for taking me under 
her wing in my first year and assisting me with the metal phosphonate project. I really 
enjoyed our golf outings as well and hope we can hit the links again sometime. Thanks 
go to Missy for her professional guidance and personal friendship. She was always there 
when my reactions failed, my personal life met a crisis, or when I just needed a trip to the 
Salty Dog to regroup. May our paths cross again. I thank Isa for all of her help with the 
AFM, SEM, and BAM experiments. To all the rest, past and present, I am lucky to have 
had the experience of working with such a professional group and value the personal 
relationships I made with all of them. 

I want to also thank my neighbors in the Boncella group for their advice, their 
chemicals, and all the laughs we shared. I graciously acknowledge Professor Katherine 
Williams and Mr. Russell Pierce for their assistance with the AA experiments. Thanks 
are also due to Professor Eric Lambers at MAIC for his help with the XPS experiments 
and Karren Kelley at the Electron Microscopy Core Laboratory for her assistance with 
the SEM experiments. The work performed by everyone in the glass, electronic, and 
machine shops, the stockroom, and those in the chemistry department staff is also 
gratefully acknowledged. 



IV 



The professional influences of my fellow co-workers are interwoven throughout 
the pages of this dissertation, but perhaps less obvious are the influences of those who 
have touched me personally, and in doing so, provided the friendship, guidance and 
inspiration which lead me to where I am today. A special thank you is given to my dear 
friend Tina Rakes for always being there when I needed her. I thank her for all the times 
she watched my son when I needed to work, for listening when I needed to talk, for 
caring, and most of all for being a friend. Her presence will be missed. Thanks also go 
to my friend Marcia Winter for cheering me up when I was down and for giving me a 
place to stay while in transition. I may not have made it through this without you. I also 
want to thank my old buddies Jason Doyle and Kirk Thrasher who were always just a 
phone call away. True friends indeed stand the test of time. 

I also want to express my deepest love and sincerest thanks to my parents for 
always being there. None of this would have been possible without their selfless 
devotion, their sacrifices, and their unending support. They taught me the value of hard 
work and wove the moral fabric of my soul. I am truly lucky to have such wonderful 
parents. I also say thanks to my brothers Chad and Luke and my sisters Cyndi and Jodi 
for always being there for me. They are not only my family; they are my friends. 

Though the course of this work, I have been influenced by many people either 
personally or professionally. My advisor, Daniel R. Talham, is one of those rare 
individuals who has influenced me in both ways. Professionally, Dr. Talham has 
provided me with exceptional guidance with my research and provided an excellent 
environment in which to learn. He has given me the direction necessary to achieve the 
goal at hand, while at the same time the necessary freedom to develop my own ideas, 



pursue my own course, and to learn by my own mistakes. I would also like to thank Dan 
personally for the tremendous support he gave me through a most difficult time in my 
life. If not for his understanding, his patience, and his inspiration my goals would have 
fallen out of reach. 

The long hours and mental devotion required for a project of this magnitude are 
perhaps felt most by those closest to you. For her sacrifices, I give Stacy my deepest 
thanks. The pressures on our relationship were more than anyone could be expected to 
bear, and I apologize for not telling her often enough how much she was appreciated. 
Her sacrifices did not go unnoticed. I also want to thank my son Garrett. He is my life, 
my love, and my inspiration. Through it all, he has been patient beyond his years and I 
am very proud of him. I can only hope that one day he will look back on this with 
understanding. Until then, we have a lot of catching up to do. 



vi 









TABLE OF CONTENTS 

Page 
ACKNOWLEDGMENTS iii 

ABSTRACT x 

CHAPTERS 

1 SUPERMOLECULAR CHEMISTRY AT INTERFACES 1 

Supermolecular Chemistry 1 

Assembling Inorganic Networks at Interfaces 4 

Thin Film Characterization Techniques 7 

Conventional Methods 7 

Characterization of Thin Films Using Synchrotron X-ray Radiation 13 

Grazing Incidence X-ray Diffraction (GIXD) 20 

X-ray Absorption Fine Structure (XAFS) 24 

2 MIXED-METAL MN-CO PHENYLPHOSPHONATES 

STRUCTURE AND MAGNETIC PROPERTIES 34 

Introduction 34 

Experimental Section 36 

Results and Discussion 38 

Sample Preparations 38 

Magnetic Properties of Mn(03PC 6 H5)H 2 and Co(0 3 PC 6 H 5 ) H 2 42 

Search for Spin Glass or Precursor Phases 49 

Negative Magnetization in the Cobalt-Rich Samples 52 

Conclusion 53 

3 STRUCTURE CHARACTERIZATION OF METAL PHOSPHONATE 
LANGMUIR-BLODGETT FILMS BY GRAZING INCIDENT X-RAY 
DIFFRACTION 54 

Introduction 54 

Experimental Section 55 

Results and Discussion 56 

Manganese Octadecylphosphonate Film 56 

Azobenzene Derivatized Manganese Phosphonate Film 58 

Lanthanum Octadecylphosphonate Film 60 

Conclusions 63 

• • 

VH 












4 FORMATION OF AN EXTENDED TWO-DIMENSIONAL 
COORDINATE COVALENT SQUARE GRID NETWORK 

AT THE AIR WATER INTERFACE 64 

Introduction 64 

Experimental Section 68 

Results 72 

Langmuir Monolayers and LB Film Transfer 72 

Spectroscopic Analyses 75 

XAFS Analysis 77 

X-ray Diffraction and GIXD 79 

Magnetic Properties 80 

Discussion 82 

Choice of System and Monolayer Behavior 82 

Structure of the Network 84 

Magnetism 85 

Mechanism and Structure Directing Elements 87 

Conclusions 88 

5 INTERFACIAL ASSEMBLY OF CYANIDE-BRIDGED 

FE-CO AND FE-MN SQUARE GRID NETWORKS 89 

Introduction 89 

Experimental Section 91 

Results and Discussion 93 

Langmuir Monolayers 93 

Infrared Spectroscopy 94 

Grazing Incidence X-ray Diffraction 96 

Magnetism 97 

Conclusions 100 

6 FERROMAGNETISM AND SPIN-GLASS BEHAVIOR IN 
LANGMUIR-BLODGETT FILMS CONTAINING A TWO- 
DIMENSIONAL IRON-NICKEL CYANIDE SQUARE GRID NETWORK 101 

Introduction 101 

Experimental 103 

Results 105 

Film Structure 105 

DC Magnetometry 106 

AC Magnetometry Ill 

Discussion 115 

Magnetic Anisotropy 115 

Spin Glass Behavior 117 

Conclusions 119 

7 SEQUENTIAL ASSEMBLY OF HOMOGENEOUS MAGNETIC 

PRUSSIAN BLUE FILMS ON TEMPLATED SURFACES 121 

Introduction 121 



vin 












Experimental 124 

Results and Discussion 127 

Film Deposition 127 

Magnetism 133 

Conclusions 137 

8 INVESTIGATIONS INTO THE INTERFACIAL ASSEMBLY 

OF LINEAR CHAIN AND 2D HEXAGONAL NETWORKS 138 

Introduction 138 

Experimental 141 

Results and Discussion 144 

Brewster Angle Microscopy 144 

Infrared Spectroscopy 146 

Grazing Incidence X-ray Diffraction 149 

Magnetism 152 

Structures of the Networks 153 

Conclusions 156 

LIST OF REFERENCES 158 

BIOGRAPHICAL SKETCH 167 



IX 



Abstract of Dissertation Presented to the Graduate School 
of the University of Florida in Partial Fulfillment of the 
Requirements for the Degree of Doctor of Philosophy 

THIN FILM MATERIALS THROUGH THE INTERFACIAL ASSEMBLY OF 

INORGANIC NETWORKS 

By 

Jeffrey Thomas Culp 
December, 2002 

Chair: Daniel R. Talham 
Department: Chemistry 

Mixed metal phenylphosphonates of composition Mn x Coi-x(03PC6H 5 ) H 2 were 
prepared with < x < 1 . The mixed metal solid solutions are homogeneous and 
isostructural with the single metal parent compounds. The magnetic phase diagram, 
down to 2 K, was constructed over the entire composition range. No evidence of spin 
glass behavior was observed for any concentration at any temperature. 

A series of Mn 2+ and La 3+ organophosphonate Langmuir-Blodgett films have 
been structurally characterized by grazing incidence X-ray diffraction using synchrotron 
radiation. The organophosphonate amphiphiles include both straight alkyl chain and azo- 
benzene groups in the organic tails. The metal oxygen networks within the films are 
found to be isostructural to related organic/inorganic layered solids. 

Reaction of a Langmuir monolayer of an amphiphilic pentacyanoferrate (3+) 
complex with Ni 2+ , Co 2+ , and Mn 2+ ions from the subphase results in the formation of 
two-dimensional cyanide-bridged network at the air-water interface. The networks can 



be transferred to various supports to form monolayer or multilayer lamellar films by the 
Langmuir-Blodgett (LB) technique. The magnetic properties of these films were 
investigated as monolayer, bilayer, and multilayer assemblies. The materials possess 
interesting physical properties such as magnetic anisotropy, magnetic ordering, and spin- 
glass behavior. 

Thin homogenous magnetic films comprised of various Prussian blue analogues 
have been prepared through the sequential absorption of the appropriate metal ions and 
hexacyano complexes onto hydrophobic surfaces that were first templated with a Fe-CN- 
Ni two-dimensional grid network deposited as a Langmuir-Blodgett monolayer. The 
films show exceptional surface coverage and magnetic behaviors similar to their solid- 
state analogues with ordering temperature ranging from 5 K to 210 K. 

The preparation of low dimensional inorganic networks through the reactions of 
Langmuir monolayers containing low symmetry amphiphilic metal complexes with 
aqueous metal cyanides was investigated. The reaction of an amphiphilic 
iron(III)terpyridine complex with aqueous Ag(CN)2 _ resulted in the formation of AgCN 
crystallites at the air-water interface as shown by grazing incidence X-ray diffraction. 
The reaction of an amphiphilic nickel(II)cyclam complex with aqueous Ni(CN) 4 2 " or 
Cr(CN) 6 " yielded cyanide-bridged products as evidenced by infrared spectroscopy; 
however, the structures of the products remain uncertain due to a lack of X-ray 
diffraction from the materials. 



XI 



CHAPTER 1 
SUPERMOLECULAR CHEMISTRY AT INTERFACES 



Supermolecular Chemistry 

The intricate complexity of biological systems can humble even the most able of 
synthetic chemists. Their vast array of structural diversity, from the molecular to the 
macroscopic level, is both aesthetically pleasing in its symmetry and awe-inspiring in its 
functional efficiency. Countless eons of trial and error have perfected synthetic 
processes wherein simple chemical building blocks self-assemble with lock and key 
precision into complex superstructures such as proteins, en2ymes, DNA and cell 
membranes. Each of these subsystems then works in tandem in intricate processes to 
create something so incredibly complex as life. Chemists have long looked to these 
natural systems for inspiration, hoping to break down and understand the underlying 
mechanisms of this process called self-assembly, with the hope of one day mastering this 
same level of synthetic control, where by simply providing the appropriate building 
blocks, complex structures with specifically tailored physical properties could be 
achieved. This synthetic paradigm of creating complex chemical systems from relatively 
simple building blocks has been termed supermolecular chemistry. l As a general term, 
the process includes synthetic techniques such as self-assembly, crystal engineering, and 
nanoscale chemistry. While having its roots in natural systems, many of the goals in this 
area of chemistry are focused on developing novel materials with molecular recognition, 
catalytic, magnetic, electrical, and nonlinear optical properties for use in separations and 



1 






nanoscale device applications. As such, it could be said that biological systems provide 
the inspiration and functional materials provide the motivation. ' 

Supramolecular chemistry as a synthetic approach takes advantage of weak to 
moderate bonding interactions between complementary molecular components to create a 
structure that is greater than the sum of its parts. The inter-molecular forces that have 
traditionally been employed are Van der Waals in nature and include pi-pi stacking, 
hydrogen bonding, and host-guest interactions. u These weaker forces allow for an 
annealing of the final structure to a thermodynamic rather than kinetic product. To 
expand the potential applications of supermolecular chemistry, many research groups 
have investigated transition metal coordination geometries as architectural driving 
forces. 4 " 13 The coordinate covalent bond has several properties that fit well with the 
supermolecular synthetic approach. The bond strength is intermediate between the 
relatively weak Van der Waals interactions and stronger covalent bonds and thereby 
offers a compromise. The bonding can now be more robust, but labile enough to allow 
for the self-annealing process that is so advantageous to self-assembled systems. Also, 
coordination complexes have well characterized, and often predictable, geometries and 
bonding angles that can aid in predicting a priori the structural motif of the final 
assembly. Supramolecular chemistry can, therefore, be reduced in complexity to a 
system in which individual linear and angular components combine into one, two, or 
three-dimensional arrays. 14 ' 15 Aside from their unique bonding and geometrical 
properties, transition metals also inherently possess useful physical and chemical 
properties such as variable oxidations states which can lead to charge transfer 



phenomena, colored materials ranging throughout the visible spectrum, cooperative 
magnetic behavior, and catalytic abilities. 

Due to the many advantages offered by transition metal complexes, numerous 
researchers have developed synthetic strategies incorporating metal ions into various 
supermolecular arrays. These strategies often involve various combinations of blocking 
ligands, bridging ligands, and complex geometries. Some of the results to date include 
"zero-dimensional" clusters, 16 " 25 polygons and polyhedra, 14 ' 15 ' 26 " 29 one-dimensional chains 
and ladders, 30 ,5 two-dimensional sheets, 36 " 44 and three-dimensional networks, 13 ' 45 " 47 
many of which possess interesting chemical and physical properties. 

One area of potential application for supermolecular assemblies is in the area of 
nanoscale devices. The electronics industry, in particular, is on a continuous quest for 
smaller, faster components, and less-expensive fabrication methods. The last few 
decades have seen a dramatic decrease in component sizes and increases in computer 
speeds, but the trend may be approaching a limit. It is generally agreed that optical 
diffraction and the opacity of lens materials or photomask supports will likely make 
current photolithographic methods ineffective for fabricating features below 100 nm. 48 
To truly break into the nanoscale regime, a new "bottom up" approach may be beneficial. 
One can envision a process where individual molecular building blocks can be assembled 
into nanoscale conductors or switches. It is by this method that the synthetic chemist 
may find application in supermolecular assembly processes. 

In addition to electronic devices, high-density magnetic storage media are also 
attractive endeavors for supermolecular chemists. Molecular magnets offer advantages 
such as size homogeneity, solubility, transparency, relatively benign synthetic conditions, 



and the potential for multifunctional materials. 49 '" 6 To date, magnetic clusters with 
significant reversal fields have been realized; however, molecular magnets with 
practically useful blocking temperatures remain elusive. 16 " 25 

The application of self-assembly methods to the fabrication of nanoscale devices 
is still in its infancy, and the level of control and complexity needed for such 
sophisticated materials has yet to be realized. However, one can argue based on the 
results reported to date that the potential exists. What is needed now is a better 
understanding of the self-assembly process and what tools the chemist can use to direct 
the architectures of the final materials. In addition, since many of the aforementioned 
applications of supermolecular systems will require the positioning, servicing, and 
interfacing of these systems at surfaces, 57 " 65 we have undertaken an investigation into the 
structure directing ability of an interface in the assembly process and what conditions 
would allow for the positioning of coordinate-covalent networks at a surface. 
Assembling Inorganic Networks at Interfaces 

From the numerous interfaces available for study, we chose the air-water interface 
as our medium in which to work. The choice was obvious for several reasons. The air- 
water interface is readily available and inexpensive; aqueous monolayers systems 
(otherwise known as Langmuir monolayers) have been well studied by our group and 
countless others over the past century; many experimental techniques are available and 
well understood for characterizing Langmuir monolayers; the final products can often be 
transferred to solid supports by the Langmuir-Blodgett technique for further 
characterizations of their structure and physical properties. 66 ' 67 

The general strategy is outlined in Scheme 1-1 . To form extended inorganic 
networks at the water surface, an amphiphilic ligand system or transition metal 



Air- water interface 



supermolecular 
networks 







Scheme 1-1. A strategy for assembling coordinate covalent networks at the air-water 
interface. The geometry of the networks can be controlled through appropriate choice of 
amphiphile, bridging species, and solute building blocks. 



complex is spread in sufficient quantity to form a molecular monolayer on the water 
surface. The subphase will be a solution of another metal complex. Either the 
amphiphilic complex or the dissolved complex will contain cyanide ligands that can act 
to bridge the two complexes together resulting in a coordination polymer at the air-water 
interface. To achieve the desired geometrical product, either the complexes themselves 
will be designed with appropriate blocking ligands attached, or the air-water interface 
will direct the two-dimensional motif by limiting reactivity in the third direction. 

The process for characterizing molecular monolayers on water surfaces by surface 
pressure vs. area isotherms was first described by Langmuir in 1917, 68 and is outlined in 
Scheme 1-2. A volatile solution of an amphiphilic molecule is spread on a water 
subphase contained in a Teflon trough. By compressing the moveable barriers, the 















amphiphiles Wilhelmy balance moveable 

\mm barrier 

i ^ 



t 



I 



compression 




L_l 



o> 
b 
3 
1/1 
(/) 

Si 
,9 



compression 
isotherm 



mean molecular area 

Scheme 1-2. A schematic depiction of the control and characterization of molecular 
order in aqueous Langmuir monolayers is shown. As the film is compressed, 
organization of the amphiphiles occurs resulting in a change is surface tension. The 
variation in surface tension is monitored by a Wilhelmy balance and is converted to a 
surface pressure vs. mean molecular area isotherm. 



effective surface area per molecule decreases and the molecules are forced to close-pack 
into a condensed state. During the compression, the surface tension as measured by the 
Wilhelmy plate varies with the degree of order in the monolayers. Extrapolation of the 
steepest part of the resulting isotherm to the x-axis gives the mean molecular area per 
molecule. In addition, a flattening of the isotherm at high pressure and smaller area is 
attributed to the collapse of the film, or the limiting mean molecular area. The behavior 






Hydrophobic 

substrate 




(submerged) 



Transferred bilayer 



Scheme 1-3. A schematic depiction of the Langmuir-Blodgett technique for preparing 
thin films for aqueous Langmuir monolayers is shown. In addition to the method shown, 
transfer can also result by starting with a hydrophilic surface submerged in the subphase 
followed with a single up-stroke. The method thus provides a unique level of control in 
preparing monolayer to multilayer thin films. 



of the isotherm can give indications as to whether the amphiphiles have cross-linked into 
a polymeric network on the water surface. Changes in the onset area, slope, and limiting 
mean molecular area between isotherms on pure water and isotherms run on metal 
containing subphases give definitive evidence for an interaction between the amphiphile 
and the subphase metal ions. 

Thin Film Characterization Techniques 
Conventional Methods 

Characterization of reaction products formed at an air/water interface presents a 
significant challenge since the quantity of material is on the order of micrograms. To 



8 









obtain further evidence of reaction it becomes necessary to collect the film onto a solid 
support. The Langmuir-Blodgett technique, as shown in Scheme 1-3, allows for the 
transfer of a monolayer from the water surface onto a hydrophilic or hydrophobic surface. 
The Langmuir-Blodgett technique is a method for preparing multilayer assemblies by a 
step-wise deposition procedure. 66-67 The film is compressed to the desired surface 
pressure and the substrate is dipped through the film into the subphase. The alkyl chains 
interact with the substrate on the down stroke and transfer to the solid support. The 
surface at the end of the dip is now terminated with the hydrophilic end of the 
amphiphiles and, upon removing the substrate, another layer of amphiphiles is transferred 
on the upstroke. The result is a bilayer of the amphiphiles held together through dipolar 
or covalent interactions between the hydrophilic head groups of the amphiphiles. The 
method also allows for the transfer of a single monolayer if the solid substrate is 
hydrophilic and the deposition procedure starts with the substrate submerged in the 
subphase. 

As alluded to earlier, the structural characterization of the sub-microgram 
quantities present in thin films presents a significant challenge. The transferred film can 
be subjected to analysis by FTIR, UV-Vis, X-ray photoelectron spectroscopy (XPS), 
atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray 
diffraction. To use UV-Vis and FTIR spectroscopies to characterize small quantities of 
materials like those obtained in monolayer reactions at a water surface, intense absorption 
processes in the product are needed. With transition metal complexes, d-d absorption 
bands are typically weak and the presence of other more intense absorption mechanisms, 



such as ligand-to-metal charge transfer, or ligand pi-pi* transitions is needed in order to 
be a useful characterization method in thin films. 

FT-ER. spectroscopy is usually quite useful in the study of thin films. The 
sensitivity of the method to submonolayer amounts of material is made possible through 
attenuated total internal reflectance crystals, hereafter named ATR crystals. These 
substrates are single crystal silicon or germanium and can be used clean as hydrophilic 
substrates or made hydrophobic by application of a layer of an alkyl silane. The 
geometry of the crystal is such that the incident IR radiation is internally reflected 
resulting in an evanescent wave that travels parallel to the sample surface. Vibrational 
modes with moderate to high absorbtivities, such as C-H, C-N, and C-0 are readily 
observed in monolayers quantities. This technique is used repeatedly thoughout the 
experiments described hereafter since the majority of the complexes involved contain the 
cyanide ligand, and cyanide-stretching vibrations are quite intense. 

Cyanide complexes can be well characterized by FT-IR spectroscopy by 
monitoring the C-N stretching frequency. These vibrations vary in frequency with 
changes in metal oxidation state and coordination environment. 69 This behavior is 
rationalized by considering the orbital interactions that are involved in transition metals 
complexed to cyanide. The cyanide anion lone pair is predominantly located on the 
carbon and nearly all of the monomelic cyano-complexes known are coordinated through 
carbon. This interaction has a strong sigma bonding interaction and effectively lowers 
electron density in the C-N antibonding orbital. As such, coordination of C-N to a metal 
center results in an increase in the C-N stretching frequency compared to the free ion. 
The C-N antibonding orbitals are low enough in energy to have significant pi-bonding 






10 

interactions with the metal center as well. The metal donates electrons to this 
antibonding orbital through a process call back-bonding. The more efficient the back- 
bonding, the weaker the C-N bond, and the lower the C-N stretching frequency becomes. 
As a consequence of these two mechanisms, a general trend is observed as one progresses 
across the first row transition metals. For trivalent metal ion complexes, the higher 
nuclear charge from chromium(III) to cobalt(III) is expected to increase the sigma 
donation and strengthen the C-N bond; however, this is nearly balanced by an increased 
ability to back-bond to the ligand, which is working to weaken the C-N bond. The result 
is that the C-N stretching frequencies remain nearly constant for the hexacyanides of 
chromium (III) through cobalt (III), occurring around 2130 cm" 1 . Another trend observed 
in metal-cyanides is that the C-N stretching frequency is lower for the divalent complex 
since the lower cationic charge decreases cyanide to metal sigma bonding and increases 
metal to cyanide back-bonding. Thus, K3Fe(CN)6 shows a C-N stretch at 2130 cm" 1 , 
whereas the same vibration occurs at 2060 cm" 1 in K4Fe(CN) 6 . A final consequence of 
the bonding interactions, and perhaps the most useful effect for characterizing polymeric 
metal cyanides, is the shift of the C-N stretching frequency when both the C and N atoms 
are coordinated. Coordination at the nitrogen end is similar in nature to the sigma 
donation of the C end, but with minimal back bonding abilities. The result, again, is that 
coordination through nitrogen removes electron density from the C-N antibonding orbital 
and strengthens the C-N bond. 70 This, combined with kinematic effects resulting from the 
increased rigidity of the structure, works to shift the C-N stretch to higher frequencies 
when cyanide assumes a linear bridging mode. 71 " 73 Therefore, this shift will be a key 






11 

signature of the formation of cyanide bridges in the structures prepared at the air- water 
interface. 

Elemental composition of thin films can be accomplished through X-ray 
photoelectron spectroscopy (XPS). X-ray photoelectron spectroscopy is a surface 
sensitive technique that distinguishes elements by the binding energies of their valence 
electrons. The sample is prepared on a conducting substrate such as silicon, and 
subjected to an electron beam of varying energy. An energy sensitive detector analyzes 
the energy spectrum of the electrons ejected from the top few layers of the sample. Peaks 
will occur in the spectrum at energies corresponding to the binding energies of the 
electrons. Since these energies are unique to the specific elements of interest, they 
provide a method for the identification of the elements present in the sample. Further 
information is obtained by integrating the peak area in combination with calculations for 
the photo-quantum yield of the transition observed. The resulting ratio of integrated peak 
areas is proportional to the ratio of elements in the sample. 

Surface morphology in thin films can be investigated by microscopic techniques 
such as SEM and AFM. Scanning electron microscopy is useful when lateral resolution 
on the sub-micron level is sufficient. For more detailed analysis where higher lateral 
resolution or accurate depth measurements are necessary, one must resort to AFM. With 
the appropriate choice of tips and scanning heads, AFM can give detailed surface 
morphology measurements with lateral resolution on the order of nanometers and depth 
resolution on the order of angstroms over sample areas varying from tens of nanometers 
to a hundred microns. Indeed, with certain systems where tunneling tips are applicable, 
lateral resolution can approach the angstrom level, giving a true atomic picture of the 



12 

surface composition. This technique is usually limited in its application for LB films 
since the surface layer is typically comprised of methyl terminated alkyl chains, which 
effectively mask the inner bilayer structure of interest. 

Conventional X-ray sources can be used to characterize the lamellar organization 
in LB films. For well-organized LB films, the system mimics a powder diffraction 
pattern of a layered system under conditions of extreme preferred orientation. The plate- 
like domains are randomly arranged in the xy-plane, but have their z-axes aligned 
parallel. As such, the diffraction pattern will consist of a series of (00/) peaks which can 
be indexed to yield interlayer lattice spacings (d) by the Bragg equation: nX = 2dsin(0), 
where 9 is one-half the angle (20) where the diffraction peak occurs. In well-organized 
LB films, the (00/) peaks can be very intense and observable with a film thickness on the 
order of 1 bilayers. Diffraction arising from the in-plane structure is typically several 
orders of magnitude less intense since there is no preferred orientation of the in-plane 
scattering vectors and acquisition times are usually too long on a conventional 
diffractometer to be of use. In addition, the random scattering from the substrate is very 
intense relative to the scattering due to the thin film sample, and a special technique 
called grazing incidence X-ray diffraction (GIXD) is required to improve the signal to 
noise ratio. This technique is best performed using high brilliance X-ray sources such as 
synchrotrons. The GIXD experiment and another thin film characterizations using 
synchrotron radiation called X-ray absorbance fine structure (XAFS) will be described in 
detail in the following section. 



13 

Characterization of Thin Films Using Synchrotron X-ray Radiation 

Synchrotron radiation 74 is produced in large particle accelerators called storage 
rings. A storage ring is a large, evacuated circular ring that may be a kilometer or more 
in circumference. It is lined with steering magnets (bending magnets) and rf generators, 
which work in tandem to keep packets of positrons or electrons circulating at energies of 
several gigaelectron volts. At these energies, the particles travel at velocities very near 
that of light. A consequence of this circular trajectory is a constant transverse 
acceleration of the packets of charged particles and a continuous emission of white 
radiation. The energy of this radiation is dependent on the energy of the particle beam 
and spans the region from ultraviolet to hard X-rays. The X-ray radiation is many orders 
of magnitude more intense than that from a typical diffractometer. Relativistic effects 
also work to force a very narrow collimation of the resulting radiation. This collimation 
makes a synchrotron source comparable to an optical laser with regard to beam 
divergence. This broad band of radiation leaves tangentially from the "bending magnets" 
in the storage ring at several different locations called beamlines. These beamlines can 
be equipped with optics and/or crystal monochrometers to select single wavelength 
radiation as necessary for individual experiments. These types of synchrotrons 
employing bending magnets are termed "second generation" synchrotrons. 

The research requiring synchrotron radiation reported in this dissertation was 
performed at the Advanced Photon Source (APS), Argonne National Laboratory, 
Argonne, IL. An aerial photograph and schematic of the facility are shown in 
Figure 1-1. Production of the beam begins with an electron gun that emits 100 keV 
electrons into a linear accelerator (linac). Here the electrons are ramped in energy by a 
series of accelerating structures to 200 MeV before striking a water-cooled 7 mm thick 



14 



-,*^ -v-js»«s^ 




■-,■.. . . 




Figure 1-1. An aerial photograph and schematic depicting the synchrotron facility at the 
Advanced Photon Source, Argonne, IL (taken from reference 75). 



tungsten disk. This interaction produces electron-positron pairs. The APS can operate 
using either electrons or positrons, but positrons are normally selected since their positive 
charge minimizes interactions with residual gas ions in the ring. The positrons are sent 
through a second stage linac that further accelerates them to 450 MeV. From here, the 
positrons are sent to a 368 m circumference booster ring where four 5-cell rf cavities 
increase the positron energies at a rate of 32 keV per turn. In 0.25 sec, the positrons orbit 
the ring 200,000 times as their energy climbs to 7 GeV. At this energy, the positrons 
have a velocity approximately equal to the speed of light. They are then injected into the 
1 104 m circumference main storage ring that is configured as a set of curves connected 
by straight sections. As the positrons change trajectory, their velocity changes, and 
radiation is emitted tangentially to the arc of the particle beam. Insertion devices, either 






15 




Figure 1-2. A schematic of a typical (undulator or wiggler) insertion device showing the 
rows of alternating pole magnets. Interaction of the radiation with the magnetic field 
results in an enhanced intensity of the X-rays through constructive interference effects. 
Synchrotrons such as the Advanced Photon Source, which employ insertion devices, are 
known as third-generation facilities (image taken from ref 75). 



wigglers or undulators, are incorporated into the straight sections and greatly enhance the 
synchrotron radiation. 

Facilities, such as the APS, which incorporate insertion devices are known as a 
"third generation" synchrotrons. These devices consist of a series of magnets oriented 
horizontally (wigglers) or vertically (undulators) to the main beam path (Figure 1-2). The 
magnetic poles alternate in polarity and introduce a sinusoidal motion to the main beam. 
This acceleration results in the emission of radiation. Undulators use a series of small 
deviations to produce an interference effect that results in nearly monochromatic 
radiation with vastly enhanced intensity. Wigglers, on the other hand, introduce a 
stronger magnetic field that destroys the sinusoidal motion via relativistic effects and 



16 






io^ 



10 



■I* 



ALSUS.O 



APSUA 
ALSUtO \ 



E W^ f ALS 

| Berting Kagtet 

frendigl 

NSLS 

Bendftg Magi* ,,,,■■—' 




3* 10 
a. 



10' 



^( 10 



10 



10 



-•"■ 



■P*" 



Carbonic; 



SSRBemf ag Magnet 



UolyfMlefiiin K 

Copper K 



Bremsstrahlung 
Coatiiuvm ,:■'■•'" 



1 



' ' | ■ ■ ' 



■1 i ■ i i ■ ■ 1 1 1 i — ■ J' i ■ ■ i J ■ > ■ i iinii 



io - 



10' io g io ' 

Photon Energy (keV) 



io' 



Figure 1-3. The X-ray brilliance of the Advanced Photon Source (APS) compared to 
other X-ray sources (taken from reference 75). Facilities such as the APS that 
incorporate insertion devices are known as a "third generation" synchrotrons. 



produces equal intensity radiation over a broad range of energies. Compared to a 
bending magnet source employing a single magnetic pole, the radiation from a wiggler is 
enhanced relative to the number of magnetic poles. The insertion devices can produce a 
power density higher than that found on the surface of the sun and a combination of flux 
and brilliance IO 4 - IO 6 greater than conventional X-ray sources (Figure 1-3). Third- 
generation storage rings maximize those x-ray beam qualities, flux and brilliance, that are 
needed for frontier experimentation. Flux and brilliance are benchmarks of x-ray beam 
quality. Both are based on a measure of the number of photons per second in a narrow 



17 

energy bandwidth and in a unit of solid angle in the horizontal and vertical directions. 
Flux is the number of photons per second passing through a defined area, and is the 
appropriate measure for experiments that use the entire, un-focused x-ray beam. 
Brilliance is a measure of the intensity and directionality of an x-ray beam. It determines 
the smallest spot onto which an x-ray beam can be focused. 

Thirty-five "sectors" are marked on the experiment hall floor. Each of these 
sectors comprises two beamlines, one originating at a bending magnet in the storage ring 
lattice, the other at an insertion device. These sectors are the domain of the APS users 
and are the locations where a wide range of experiments take place. The beamline in 
which the University of Florida has an interest is maintained by a multi-institutional 
consortium call the Materials Research Collaborative Access Team, or MRCAT, and is 
located at sector 10ID. 76 

The MR-CAT undulator line is fully operational. As described earlier, an 
undulator insertion device provides enhanced X-ray intensity over a narrow energy range. 
Experimenters have control over the undulator and can tune the energy range of the 
output by adjusting the "gap" between the magnetic poles. The larger the gap, the wider 
the energy range and lower the intensity of the resulting radiation. Typically the gap is 
set to 100-200 eV to compromise intensity and beam stability. Final selection of 
monochromatic radiation over an energy range of 4.8-30 keV is accomplished by a cryo- 
cooled Si (1 1 1) double crystal monochromator designed by the IIT Center for 
Synchrotron Radiation Research. The optimum energy range is between approximately 6 
keV and 13 keV, since lower energy radiation is strongly scattered by air and higher 
energies require operation with crystal harmonics. As such, operations outside of the 



18 

optimum range, while possible, are less trivial and require greater attention to 
experimental parameters. The second crystal has a piezoelectric tuning actuator with a.c. 
feedback and a Bragg-normal motion that permits some degree of fixed-offset operation. 
The monochromator assembly is housed in a separate hutch (called the A-hutch) that is 
upstream from the main experimental station. 

The term monochromator is somewhat of a misnomer since the radiation coming 
off the crystals contains harmonics due to higher order Bragg reflections. This is 
compensated for inside the main experimental hutch (called the B-hutch) by reflecting the 
beam off a flat, 60 cm long harmonic rejection mirror with Pt and Rh coatings. By 
adjusting the tilt of the mirror, the lower energy radiation can be selectively reflected 
while the higher energy harmonics are unaffected. This angle can be found by scanning 
the mirror tilt and monitoring the intensity of the reflected beam. The intensity will 
remain relatively constant until the angle passes a critical angle where the intensity drops 
rapidly. By setting the mirror tilt at a value just prior to the cut off angle, higher energy 
radiation is effectively discriminated since the angle at this position is above the critical 
angle for the higher energy components. The profile of the beam coming off the mirror is 
ovular with a width of- 5mm and a height of ~ 3 mm. The intensity of the beam varies 
over this area, with the most intense section in the center of the beam. A homogeneous 
section is selected by the use of motorized slits with a motor resolution on the order of 
microns. 

The presence of a vertically deflecting mirror at the front of the experimental 
station requires all optical components to be mounted on an incline in order to be inline 
with the X-ray beam. This is accomplished by use of an X95 rail system, which also 



19 

performs the function of standardizing all component mounts to one of two rail-to-beam 
distances. A second float glass mirror is available for use as a steering mirror for liquid 
scattering experiments. This allows for a more facile transition between experimental 
operations. 

The X-rail terminates down stream at an 8-circle Huber goniometer that is 
mounted on a large positioning table. The table can be moved vertically and horizontally 
perpendicular to the beam to synchronize the goniometer center to the beam position. 
Two of the 8 circles that control the detector position have encoded motors that permit 
continuous scanning and data acquisition using the multichannel scaler described below. 

Several detector types are available for use in MR-CAT sector. High flux 
measurements, such as those required for incident beam monitoring, are accomplished 
through Daresbury design spectroscopy ion chambers for use on the main X95 
spectroscopy rail or with smaller Cornell-type ion chambers that may be mounted on the 
spectroscopy rail or on the Huber goniometer detector arm. Lytle-type fluorescence 
detectors are also available and typically used in XAFS spectroscopy. The goniometer 
detector arm can be fitted with sensitive Nal scintillation detectors for high-resolution 
diffraction experiments. Data collection is accomplished through an instrument chain of 
Keithley electrometers, V-F converters and a 32 channel multi-channel scaler. The multi- 
channel scaler permits the simultaneous monitoring of the monochromator energy, 
detector outputs, and the goniometer detector motor positions for use in slew scan 
operations. Motor commands and data acquisition are handled by the MX system, 
written by William Lavender. 77 









20 

harmonic 

rejection 

mirror definm ^ Qm scintillation 

steering ... ^ counter 

slits monitor »»«••» • 

mirror 




X-rays from 

monochomator ' — " 



B-B 



sample 
surface 



Figure 1-4. Schematic showing the experimental set-up for GIXD experiments. The 
sample surface can be either a solid support or a water surface. The steering mirror is 
float glass coated with a platinum strip and controls the incident angle to the water 
surface. For solid samples, the steering mirror is unnecessary since the incident angle 
can be controlled by the sample position. 



Grazing Incidence X-ray Diffraction (GIXD) 

To obtain X-ray diffraction patterns of monolayers requires a specialized 
technique developed over the past two decades (Figure 1-4). The technique is called 
grazing incidence X-ray diffraction (GIXD) and requires intense X-rays from synchrotron 
sources. The theory behind the technique has been described in detail and only a brief 
outline will be presented here. 78 

The theory behind the technique is based on well-known laws of optics that 
govern the interaction of electromagnetic radiation with interfaces consisting of a change 
in index of refraction (n). A wave incident on a flat interface at some angle a; is both 
refracted into the second medium and reflected off the surface. As a* approaches some 
critical angle, etc , the angle of the refracted wave with respect to the surface approaches 
zero and the incident wave is totally reflected. At incident angles just below the critical 
angle, the incident wave vector factorizes into two components, one horizontal and one 
vertical. This horizontally propagating (evanescent) wave has an exponentially 
dampened amplitude along the surface normal and a resulting penetration depth that 






21 






varies as a function of the incident angle. At X-ray wavelengths, the penetration depth in 
water when cii= oc c is approximately 100 nra, but decays rapidly to approximately 5 run 
when a* = ('/2)a c . The critical angle for water, at X-ray energies on the order of 1 keV, 
is ~ 2.4 mRad. At this angle, the incident radiation has become surface sensitive. At 
a.i= etc, the evanescent and incident waves are in phase and the amplitude of the 
evanescent wave is effectively doubled. Since intensity is the square of the amplitude, 
the evanescent intensity is quadrupled at the critical angle. As the incident angle 
decreases below the critical angle, the incident and evanescent waves become more and 
more out of phase and the intensity of the evanescent wave decreases. In a typical GIXD 
experiment, the X-rays are incident on the water surface at ~ 0.85ac, which is an effective 
compromise between keeping the X-rays surface sensitive and keeping the evanescent 
intensity high. This same approach applies to LB films transferred onto solid supports as 
well. 

This experimental technique has been readily applied to the structural 
characterization of aqueous Langmuir monolayers and LB films. 78 " 80 In all cases of 
crystalline films, the materials are found to behave as 2D powders. That is, there is no 
preferential orientation of Bragg planes and the crystals are randomly dispersed through 
rotational disorder about the z-axis perpendicular to the plane of the film. While these 
two-dimensional powder patterns do not provide the detail available in traditional single 
crystal diffraction experiments, knowledge of the limited structural motifs possible for 
the packing of alkyl chains combined with lattice-energy calculations and the information 
derived through GIXD experiments can provide a structural picture of the LB film in 
surprising detail. 78 " 80 



22 

In the simplest model, a domain of the film is treated as a two-dimensional crystal 
consisting of uniformly oriented rigid molecules. The scattering pattern is then governed 
by the structure factor reflecting translational order of the molecular centers in the plane 
of the monolayer and the form factor of the individual molecules. The translational order 
within the xy-plane will give rise to a diffraction peak at 26 when the (h k) lattice planes 
make an angle 0hk with the evanescent beam fulfilling the Bragg condition X = 2dsin(0hk)- 
Unlike a three-dimensional crystal, there is no restriction on the z-component of the 
scattered beam. The Bragg scattered beam may go into the water or exit the surface at an 
angle Z . As a result, the 2D lattice confines the scattering vectors to Bragg rods instead 
of points. The Bragg rod will have an intensity profile, due to interference effects 
between the scattered wave and the wave reflected from the surface. The two waves will 
be in-phase when the scattered angle, 9 Z , matches the critical angle, etc, and a maximum 
will occur in the Bragg rod profile. At higher angles, the two waves become out of phase 
and the intensity decays. Coupled with this phenomenon, is the interaction of the 
molecular form factor with the Bragg rod profile. The structure factor for a rod like 
molecule is large only on a plane perpendicular to the rod long axis. The intersection of 
this plane with the Bragg rods will give rise to diffraction maxima. 

Analysis of the GIXD patterns for Langmuir monolayers has been described in 
intricate detail and the following brief description of the method is heavily borrowed 
from these references. Close-packed alkyl chains, assuming rotational symmetry, can 
basically adopt three packing motifs: hexagonal, centered-rectangular, and oblique. If the 
molecules are all standing perpendicular to the film plane and the packing has hexagonal 
symmetry; in rectangular notation, the three lowest order peaks (0 2), (1 1) and (1 -1) are 



23 






degenerate (the (1 0) and (0 1) reflections are absent due to symmetry), and the Bragg rod 
will have its maximum at Z = 0. If the molecules are all standing perpendicular to the 
film plane, and the unit cell stretches or shrinks towards a nearest neighbor, the cell is 
centered-rectangular. The (0 2) degeneracy is removed and two peaks with an 
approximately 2: 1 intensity ratio will be observed in the xy-plane. If the unit cell 
stretches towards a nearest neighbor, the (0 2) peak will be at a larger angle than the (1 1) 
(1-1) degenerate peak. If the cell shrinks, the opposite inequality is observed. 
Symmetry can also be removed by tilting of the alkyl chains. If the alkyl chains tilt in a 
nearest neighbor direction, the cell is distorted to centered-rectangular and one of the 
degeneracies is removed. The result again is that two peaks with an approximately 2: 1 
intensity ratio will be observed in the xy-plane. The Bragg rods for the two peaks will 
have different intensity profiles with the non-degenerate peak having its maximum at 9 Z = 
0, and the degenerate peak will have its maximum at 6 Z > with 6 Z dependent on the 
magnitude of the molecular tilt. To calculate the tilt angle, it's more convenient to plot 
the diffraction data relative to the incident wavevector "K" where K XT = (47t/A.)sin9 xy and 
K z = (27t/X)sin8 z . Now the tilt angle § can be calculated via the relationship 
tan <f> = Kjz / [(K dxy ) 2 -[( 1 /2)(Knxy)] 2 ] 2 where K^ is the K z value for the degenerate 
reflection, K<jxy is the K X7 value for the degenerate reflection, and Knxy is the K xy value for 
the non-degenerate reflection. Similar geometric arguments can be applied to calculate 
tilt angles when the alkyl chains tilt towards a next-nearest-neighbor, or in an 
intermediate direction. 79 

For the systems described throughout this dissertation, the analysis of the 
diffraction data is concerned primarily with the structure of a two-dimensional inorganic 



24 

network contained within the Langmuir monolayer or Langmuir-Blodgett film. The 
structure of the organic chains are of secondary importance since the rigid nature of the 
inorganic lattice will most often be incommensurate with the alkyl chain packing. The 
result of such a mismatch will be either a disordered arrangement of the organic chains or 
the formation of small aggregated domains, neither of which are well suited to fulfill the 
conditions for high quality diffraction. The interpretation of the diffraction data can also 
be more complex for inorganic systems, since the number of packing arrangements and 
possible lattices are much larger than in simple rod-like hydrocarbon chains. The Bragg 
rod profiles for planar inorganic networks will have their maximum intensity at the 
horizon due to the finite thickness of the quasi-two-dimensional system and as such yield 
little structural information. Interpretation of the in-plane diffraction pattern is made 
somewhat simpler by the reduction in Miller indices to two values for a planar lattice. 
Additional information can also be obtained through systematic absences present in the 
indexed diffraction peaks that result from crystal symmetries. However, a complete 
solution to the structure is still difficult if not impossible due to the infinite possible 
arrangements of the constituent atoms. The best approach is to use comparisons to 
analogous solid-state structures in combination with a chemical intuition of typical 
bonding arrangements and complex geometries to arrive at a structure that agrees well 
with the diffraction data. 
X-ray Absorption Fine Structure (XAFS) 

Complementary to x-ray diffraction is another technique useful in the structural 
characterization of materials. The technique is x-ray absorption fine structure 
(XAFS). ' When a collimated beam of monochromatic radiation travels through matter 
of thickness x, its intensity decays according to I / I Q = e***, where !<, and I are the 



25 




r* - 






Eo 



*C fl •* 

»;-r ■■•■"« 



>4U~)< 



\ 



V 



KXAFS 



Figure 1-5. The absorbance coefficient for a typical substance at X-ray wavelengths 
spanning an adsorption edge energy (E ). The fine structure is the result of interference 
effects between the propagating and backscattered photoelectron waves. The fine 
structure in the post edge region is the phenomenon known as XAFS (figure taken from 
reference 82). 



incident and transmitted intensities and u. is the linear absorption coefficient. As the 
energy of the photons is increased, u generally decreases until a critical energy is reached 
where the absorption suddenly increases several-fold. This discontinuity is referred to as 
an absorption edge and relates to the ejection of a core electron from an atom to a 
continuum state. Further increasing the energy will cause a further decrease in u until 
another absorption edge is encountered. These absorption processes are designated as K, 
L, M, etc. according to the orbital state from which they originate. There is one 
absorption edge for the K shell, three for the L shell, five for the M shell, and so on. 

Closer inspection of the X-ray absorption spectrum for materials often reveals a 
fine structure in the fix vs. E plot in the pre-edge and post-edge regions, Figure 1-5. 
Peaks in the pre-edge region arise from absorption processes involving the excitation of 
core electrons to bound states and can provide insight into bonding information such as 



26 

energetics of virtual orbitals, electron configuration, and site symmetry. Oscillations in 
the region 40-1000 eV beyond the absorption edge arise from final state interference 
effects involving scattering of the outgoing photoelectron from the neighboring atoms. 
These oscillations are the XAFS for X-ray absorption fine structure (also called EXAFS 
by some authors). In between the pre-edge and post-edge regions is the X-ray absorption 
near edge structure (XANES) that arises from effects such as many-body interactions, 
multiple scattering effects, distortion of the excited state wavefunction by the coulomb 
field, band structures, etc. 

Since XAFS is an effect arising from the interference of the outgoing 
photoelectron wave from the absorbing atom with an incoming wave backscattered from 
a neighboring atom, then a detailed analysis of this interference effect could provide 
details about the local structure surrounding the absorbing atom. Indeed, this is the value 
of XAFS spectroscopy. As a local probe, XAFS can complement X-ray diffraction data 
since diffraction arises from a periodic arrangement of atoms over a large lattice and 
XAFS focuses on the coordination shell within 10 A of the absorbing element. Of 
course, XAFS cannot compete with the amount of information available through single 
crystal diffraction where complete structure solutions are derived. But with samples 
where single crystals are unavailable, XAFS can offer a unique probe into local 
environments that may not be obtainable by other methods. For example, XAFS is 
applicable to gasses, liquids, solutions, and crystalline or amorphous solids. In addition, 
since XAFS results from an absorption process, it is element specific. Although the 
XAFS phenomenon and its basic explanation in terms of quantum mechanical 
interference effects have been known since the 1930, the phenomenon did not become a 



27 

practical experimental tool until Sayers, Stern, and Lytle distilled the essential physics of 
the process into the standard XAFS equation and proposed a simple method of data 
analysis. 83 This achievement, coupled with the availability of tunable, high flux, high 
energy-resolution synchrotron facilities, has led to an exponential growth in the number 
of XAFS experiments performed since 1970. 

The interpretation of an XAFS spectrum, %(E), acquired as a function of 
energy, first involves normalization to the background absorption (u«) by 

X(E) = [ [ i(E)- [ i (E)]/ [ i (E) [1-1] 

followed by the conversion from E space to k space via the relationship 

k = [(8n 2 m I h 2 )(E - E )] * [ 1 -2] 

from which structural parameters can then be determined by application of the standard 
XAFS equation 

X(k) = T^iH^jky-^Wp-^'Hk) sin(2fr, + Mk)) 

j kS 

[1-3] 

Here Fj(£) is the backscattering amplitude from each of the Nj neighboring atoms of the 
y'th type with a Debye- Waller factor of Oj (to account for thermal vibration and static 
disorder). The total phase shift of the photoelectron is <})„(&) and contains contributions 
for the absorbing atom / and the backscattering atomy. The term e' 2r > ' H k ~) takes into 
account inelastic losses in the scattering process with Aj being the electron mean free 
path, and the amplitude reduction factor, Si(A), takes into account inelastic losses due to 
multiple excitation. Thus, each XAFS wave is determined by the backscattering 
amplitude (NjFj(*)) modified by the reduction factors Sj(/t), e' 2 ^, and e'VW, the 1 / krf 



28 

distance dependence, and the sinusoidal oscillation which is a function of interatomic 
distances (2/tj) and the phase shift (<t>ij(£)). 

A theoretical discussion on the origin of these parameters and their relevant 
effects on XAFS spectra is beyond the scope of this introduction; 81 * 82 however, a few 
general points can be made. Since the sinusoidal XAFS oscillation results from the 
interference s\n(2kr) term, with a frequency 2r in k space, the more separated the 
absorbing atom and backscattering atom (larger r) the higher the frequency of the 
oscillation. The intensity of a spherically propagated wave decreases as 1/r 2 , so the 
XAFS amplitude decays rapidly with distance. While the amplitude function Fjljc) 
depends mainly on the type of backscatterer, the phase function contain contributions 
from both the propagating atom and the backscattering atom. It should also be mentioned 
that the standard XAFS equation is an approximation based on the assumption that the 
process results from a single-scattering event. This is normally a valid assumption since 
multiple-scattering pathways are merely a sum of all the scattering pathways that 
originate and terminate at the central (absorbing) atom. As such, the path lengths are 
typically quite large resulting in significant attenuation and high frequency oscillations 
that tend to destructively interfere. Multiple-scattering can become important when 
atoms are aligned in a collinear array. In these types of systems, the propagated wave is 
strongly forward- scattered by the intervening atom, resulting in significant amplitude 
enhancement. This process is most evident in systems where the bond angles are 1 80 ± 
30°. In these cases, modifications to the standard XAFS equation are necessary to take 
into account the multiple scattering processes. 



29 



synchrotron 
source 




harmonic 

rejection 

mirror 



i-;::«( 






monochromator 




S 



sample 



Z - 1 filter/slits 



I, 

monitor 



reference 
foil 



Figure 1-6. Schematic of an experimental setup for collection XAFS spectra in 
fluorescence mode. 



The discussion to this point has centered on XAFS taken in absorbance mode, but 
the technique is also applicable in fluorescence mode. In fact, for thin film samples, such 
as those obtained by LB methods, the absorbance due to the sample is negligible and data 
can only be collected as fluorescence. A schematic of a typical experimental 
arrangement for collecting fluorescence XAFS spectra is shown in Figure 1-6. The 
source radiation is scanned over the required energy range by movement of the 
monochromator and reflected off a harmonic rejection mirror located inside the 
experimental hutch. The incident flux is monitored by a ion chamber positioned 
upstream from the sample-fluorescence detector assembly. The sample is then oriented 
45° relative to the incident beam. The fluorescence detector (Lytle detector) is then 
placed at 45° relative to the sample with a Z-l filter placed between the sample and Lytle 
detector. There is also a set of collimators oriented between the sample and Lytle 
detector to minimize randomly scattered X-rays from reaching the detector. A thin foil of 
the same element to be measured in the sample is position behind the sample housing and 
in front of a second ion chamber so that a reference absorbance edge can be measured in 
situ. This reference foil provides a convenient method of energy calibration since the 
absorbance edge is known to within a fraction of an eV for most elements. The measured 



30 

fluorescence spectrum can then be converted to an experimental spectrum u.(E) where the 
y-axis is the total linear absorption coefficient and the x-axis is energy. For fluorescence 
experiments, u,(£) is calculated by u(£) = F 1 1 where F is the measured fluorescence 
intensity and I is the measured incident X-ray intensity. Prior to the experiment, the 
linearity of the detector responses should be verified. This can be accomplished by 
monitoring the ratio F 1 1 with full 7 and after attenuation with appropriate filters to 
(V2)I . If the detectors are linear, the ratio ofF/I will not vary by more than a few 
percent. If the response is not linear, then the sensitivity of the detectors should be 
adjusted by varying the detector gasses. To minimize background scattering from the 
sample support, the sample should be prepared on an "X-ray transparent" support such as 
Mylar. 

An XAFS scan normally involves scanning the energy from -150 eV before the 
edge to -1000 eV past the edge. The scan step size can be varied to give a higher 
resolution in the vicinity of the edge (~ leV / step) and a slightly lower resolution after 
the edge (~ 3-5 eV / step). In addition, multiple scans can be taken and averaged for a 
better signal to noise ratio. 

Once the data has been collected, the process of extracting the important 
structural information from the XAFS spectrum can begin. This process is greatly 
simplified by application of an appropriate software package such a WinXAS 84 that has 
been designed specifically for XAFS data reduction. In addition, the WinXAS program 
has been designed to accept inputs from modeling programs such as FEFF7 85 which can 
calculate theoretical XAFS parameters based on atomic coordinates prepared via the 
input program ATOMS. 86 Using these three programs in tandem, one can efficiently 



31 

perform the background subtractions, Fourier transformations, and curve fitting routines 
necessary for interpretation of the XAFS data. 

The first step in the process of data reduction is the removal of a "raw 
background" which is normally present as a smoothly varying, low-order polynomial 
evident in the pre-edge and post-edge data curve and the normalization of the data to the 
edge step. In the WinXAS program, these steps can be preformed in one step by simply 
fitting the pre-edge (typically linear) and post-edge data (typically a second order 
polynomial) to two separate functions and subsequently subtracting them off. The next 
step is to convert the \i(E) from E space to k space. In order to do this, the edge energy 
(E ) must be determined. With WinXAS, this is done by finding the inflection in the 
edge step using a second derivate curve. Once (E ) has been determined, the conversion 
to k space can proceed by simply selecting the conversion step (with the proper (E ) 
input) with the WinXAS program. The data at this stage still contains other "background 
factors" such as spectrometer baseline, beam harmonics, elastic scatterings, etc., which 
will are removed by fitting to a cubic spline function. The power and number of nodes in 
the spline function can be varied to get the best fit. The fit window is varied to provide 
the best result and is typically in the range 2.5 < k < 11. Quality of fit can most easily be 
judged by monitoring the Fourier transformed "radial plot" which varies in real time in 
WinXas as the fitting window is varied. The main objective is to minimize peaks below 
one angstrom in the radial plot and to ensure that the radial plot remains relatively 
constant as the fitting window is changed. It is best to try several different cubic splines 
to determine which gives the best fit. 



32 

Once the background has been subtracted; the final stage is the Fourier transform. 
This is best done in combination with a Bessel window function that minimizes the high 
frequency ripples that result from the finite size of the transform data. It is also 
advisable to truncate the data window at values that give the smoothest continuation from 
one end of the data window to the next. Once the window has been chosen, the Fourier 
transform proceeds by opening a separate window where the results are displayed for 
four different values of k weighting. Typically for transition metals, it is best to use a k 3 
weighting factor which amplifies the lower intensity high k data, giving better results. 

The Fourier transform now yields the "radial plot" which consists of a series of 
peaks corresponding qualitatively to different coordination shells. The peak positions are 
not absolute and are offset by a phase shift. To fit the radial data, one must construct a 
model of the coordination environment using the program ATOMS. The important data 
here are atom type, coordination number, bond angles, and bond lengths. These values 
are read by the input program FEFF and imported into WinXas as the starting points for 
the fitting routine. The major problem with XAFS fitting is now apparent, as there are 
several fitting parameters in the XAFS equation to vary. The theoretical values 
calculated by FEFF are quite accurate, but if at all possible, model compounds should be 
run with known structures and similar bonding interactions to extract expected variables 
such as the Debye Waller factor and edge energy shifts. For the FeNi grid network, 
values were obtained from FeCo Prussian blues reported in the literature. With each 
variable having a starting value and expected range, the fitting can begin. In the 
beginning, hold different variables constant, and vary each individually to see the effects 
of each variable, then steadily progress including more variables until a good fit is 



33 

obtained. With a large number of variables, the fit is not absolute, but if the modeled 
cluster gives a good quality fit with reasonable values of the non-structure variables then 
the model cluster is well supported. 









CHAPTER 2 

MIXED-METAL MN-CO PHENYLPHOSPHONATES 

STRUCTURE AND MAGNETIC PROPERTIES 



Introduction 

Even before Clearfield's elucidation of the structure of the prototype 

a-Zr(HP04)2 H2O, layered metal phosphates were extensively studied primarily because 
of their ion exchange capabilities. 88 This initial interest has been extended to metal 
phosphonates where similar architectures are found 89 " 93 and now includes organic 
networks that can be varied to further modify the properties of the layered solids. 94 " 95 
Recently, layered metal phosphates and phosphonates have been shown to exhibit 
interesting magnetic phenomena, including magnetic ordering, canted antiferro- 
magnetism 100 " 107 and antiferromagnetic resonance, 108 and they have been studied as 
models for two-dimensional (2D) magnetism. Our group has also extended these studies 
from the solid-state, 109 to monolayer 110 " 112 and multilayer thin films, 113 ' 119 where similar 
properties have been observed. 

As part of our interest in 2D magnetism in metal phosphonate solids and thin 
films, we have investigated a series of mixed-metal Mn 2 7Co 2+ and Mn 2+ /Zn 2+ 
phenylphosphonates. Two possibilities exist if mixed metal phases form, each giving rise 
to different magnetic behavior. If ions of a different spin state organize in an ordered 
fashion, then a new superstructure is formed giving rise to the possibility of 
ferrimagnetism if the spin state of the two ions is different. Alternatively, if the ions 



34 



35 

distribute randomly, then a solid solution results. Historically, mixed metal solid 
solutions have been extensively studied 120 because they exhibit altered magnetic behavior 
and provide an opportunity to study the details of magnetic ordering mechanisms. 
Systems based on Mn 7Co + have been popular choices, as the materials cover a range of 
dimensions, from quasi-lD 121 " 123 to quasi-2D 124 to 3D, 125 and in most cases an isotropic 
(Heisenberg-type) interaction describes the coupling between S = 5/2 spins of Mn(II) 
ions, while an anisotropic (Ising-type) interaction describes the coupling between 
"effective" S = 1/2 spins of Co(II). Consequently, upon dilution these materials 
experience an interesting blend of competing spin and lattice dimensions. Despite 
previous studies on mixed-metal solids, there are still some unanswered questions. For 
example, in some cases, but not all, the combination of random mixing and magnetic 
frustration leads to spin glass behavior. 126 In addition, some ferrimagnetic systems have 
exhibited the interesting effect of negative magnetization. 127128 New examples of mixed- 
metal magnetic systems, either structurally ordered or as solid solutions, can provide the 
opportunity to further study some of these phenomena. 

The divalent metal phenylphosphonates form an isostructural series 
(Figure 2-1), and we find that the mixed-metal analogs form as solid solutions of 
formula MnxCoi.^OaPCeFtyl^O or Mn x Zn,. x (0 3 PC 6 H5) H 2 0. At low temperature, the 
pure Mn + and pure Co + phenylphosphonates experience long-range antiferromagnetic 
order at T N « 12 K and 4 K, respectively. Upon dilution, the ordering temperatures are 
reduced compared to the values found for the pure compounds, and the resulting 
magnetic phase diagrams are reported here. For diamagnetic Zn 2+ doping, i.e. Mn x Zni. x , 
the reduction of T N follows the prediction of mean field theory for x > 0.6 and this 



36 





Figure 2-1 In-plane and cross-sectional view of Mn(0 3 PC6H 5 )H 2 0. Crystallographic 
data are taken from reference 5. Key: oxygen, small open circles; manganese, 
crosshatched circles; phosphorus, diagonal-hatched circles (phosphorus atoms above and 
below the plane are distinguished by hatches with different directions). 



magnetic phase diagram was reported previously. 129 However, for the Mn x Coi. x 
compounds, the reduction of T N with doping concentration is weaker than expected on 
the basis of mean field theory. For Mn x Coi. x at low temperatures, the magnetization of 
the Mn-rich specimens, i.e. x > 0.25, is characterized by canted antiferromagnetic 
behavior. On the other hand, the magnetization of the Co-rich specimens, i.e. x < 0.25, 
exhibits a very small negative magnetization behavior when the zero-field cooled and 
field cooled data are compared. The magnetic phase diagram for Mn x Coi. 
x (0 3 PC 6 H 5 ) H 2 is reported here. 

Experimental Section 
Materials used. Reagent grade Mn(N0 3 ) 2 4H 2 0, CoCl 2 6H 2 and 

phenylphosphonic acid (C6H 5 P0 3 H 2 , 95%) were purchased from Aldrich (Milwaukee, 

WI) and used without further purification. The water used in all reactions was purified 

with a Barnstead NANOpure purification system that produced water with an average 

resistivity of 18 MD cm. Mn(0 3 PC 6 H 5 )H 2 and Co(0 3 PC 6 H 5 ) H 2 were synthesized by 



37 

mixing equimolar amounts of the appropriate metal ion solution with a solution of 
phenylphosphonic acid (pH adjusted to 5-6 with 0. 1 M KOH) both heated to 60°C prior 
to mixing. The solutions were allowed to stir for two hours at this temperature. For each 
sample, the precipitate was filtered, washed with water and subsequently with acetone, 
and then dried under vacuum. 

Preparation of MnjCoi.,(03PC6H 5 ) H 2 compounds. The mixed-metal 
phenylphosphonates Mn x Coi-x(03PC6H 5 ) H 2 were prepared in a manner similar to the 
pure metal phenylphosphonates but with slight modification. In each case, aqueous 
solutions of the metal salts in the desired molar ratios were heated to 60°C and added to a 
solution containing a slight excess of phenylphosphonic acid at pH 5-6. The resultant 
solutions were stirred for only 10 minutes before filtering the precipitate. The products 
were washed with water and acetone, and finally dried under vacuum. In all cases, the 
final Mn:Co ratios (determined by atomic absorption) of the solid-state materials were 
similar, i.e. within 10 %, to those of the starting metal salt solutions. 

Instrumentation. Atomic absorption (AA) measurements were performed on a 
Perkin-Elmer Model 3 100 atomic absorption spectrometer with a photomultiplier tube 
detector. For AA analysis, the solid-state samples were dissolved in a 1 .0 M HC1 
solution. X-ray diffraction was done with a step scan (0.02° 20/step, 2 sec/step) using a 
Phillips APD 3720 X-ray powder diffractometer with the Cu Kq- line as the source. 

Electron paramagnetic resonance (EPR) spectra were recorded on a Bruker (Billerica, 
MA) ER 200D spectrometer modified with a digital signal channel and a digital field 
controller. Data were collected using a U.S. EPR (Clarksville, MD) SPEC300 data 
acquisition program and converted to ASCII format using a U.S. EPR (Clarksville, MD) 



38 

EPRDAP data analysis program. Magnetization and AC susceptibility measurements 
were performed using a Quantum Design MPMS SQUID magnetometer. The DC 
measurements were made with a measuring field of 100 G or 1 .0 kG when sweeping the 
temperature, or were made at 2 K while sweeping the field up to 50 kG The AC 
susceptibility measurements used frequencies ranging from 17 Hz to 1.5 kHz and an AC 
field amplitude of 4.0 G. Additional low frequency (19 Hz) AC susceptibility 
measurements were performed with a homemade mutual inductance coil of a standard 
design. High frequency (14 MHz) studies were conducted in a homemade tank-circuit 
biased with tunnel diode. 130 ' 131 For all of the magnetic studies, powder samples were 
contained in gelcaps or plastic vials, with the exception of the work performed at 14 MHz 
when the sample was loaded directly into the housing of the coil. The background 
signals arising from the gelcaps and vials were independently measured and were either 
negligible or subtracted from the data. 

Results and Discussion 
Sample Preparations 

In order to encourage homogeneous solid solutions of composition 
Mn x Coi. x (0 3 PC 6 H5)H20, and to prevent any annealing into a multi-phased system, 
samples were quickly precipitated and collected immediately. This procedure resulted in 
a decreased crystallinity of the solid solutions, relative to what is possible with the pure 
phases, although it is sufficient for powder XRD analyses and does not appear to 
influence the magnetic properties. All attempts to prepare mixed-metal samples of high 
crystallinity by slow growth techniques resulted in the formation of physical mixtures 
and/or multi-phase materials. 



39 



Table 2-1. Concentration of manganese in MnxCol-x(03PC6H5).H20 determined from 
AA spectroscopy and unit cell parameters determined from the 1 10, 01 1 and 030 hkl 
reflections in the corresponding powder XRD patterns. 












Mol % Mn 


a+0.01 


b±0.01 


c+0.01 


100 


5.73 


14.34 


4.94 


95 


5.73 


14.34 


4.94 


82 


5.70 


14.34 


4.92 


68 


5.68 


14.34 


4.90 


55 


5.66 


14.34 


4.88 


35 


5.65 


14.34 


4.88 


30 


5.64 


14.34 


4.87 


21 


5.62 


14.34 


4.86 


19 


5.63 


14.34 


4.85 


11 


5.61 


14.34 


4.85 


10 


5.61 


14.34 


4.85 





5.60 


14.34 


4.83 



Structural Characterizations 

The relative percentage of manganese and cobalt in the solid solutions was 

determined from AA analyses (Table 2-1). Although AA spectroscopy gives an average 
stochiometry, it cannot provide information about the structural homogeneity of the 
samples. Therefore, X-ray diffraction was used to determine if the final product consists 
of single or multiple phases. The structures of the pure manganese and cobalt 
phenylphosphonate compounds consist of layers of quasi-two-dimensional metal- 
phosphorus-oxygen sheets that define the ac plane, while the organic moieties project 
between the layers thus defining the 6-axis (Figure 2-1). 90 ' 91 These materials are known 
to crystallize in the same space group, Vmn2 u with slight modifications of the ac basal 
plane spacings. 90 " 91 However, the inter-plane distances are almost identical because both 
compounds contain the same organic phenyl group. 



40 



- 



e 
s 
o 
U 



o 
U 






— I 1 1 1 1 ' 1 ' 1 ' 1 - 

16 20 24 28 32 36 

2© (degrees) 



~i ' r 

18 19 

20 (degrees) 




Figure 2-2. A. X-ray powder diffraction patterns of a: Co(0 3 PC6H 5 )H 2 0,b: 

Mno 35Coo.65(0 3 PC6H5) H 2 0, c: Mn(0 3 PC 6 H 5 ) H 2 0. B. Expansion showing, from left, 

1 10, 030, 01 1 hkl reflections for a: Co(0 3 PC 6 H 5 ) H 2 0, b: Mno. 3 5Coo.65(0 3 PC 6 H5)H 2 0,c: 

Mn(0 3 PC 6 H 5 )H 2 0. 



Due to the symmetry of the Vmn2\ space group, the 100 and 001 reflections are 
systematically absent, so the highest order reflections containing in-plane structural 
information are the 1 10 and 011. Although both of these reflections contain an inter- 
plane contribution, this distance remains essentially constant for all compositions. The 
1 10, 030, and 01 1 reflections conveniently occur consecutively over a small range of 
29 in the X-ray diffractogram, making them a practical series for monitoring variations in 
the ac lattice spacings. The position of the 030 reflection in all samples is an internal 
reference that confirms that the inter-plane distances do not change as a function of 



41 

doping, allowing the 29 values for the 1 10 and 01 1 reflections in the doped materials to 
be used to determine the in-plane lattice spacings. 

Powder XRD patterns for the pure manganese and pure cobalt phenyl- 
phosphonates, as well as that of the Mno.35Coo.65 sample are shown in Figure 2-2. The 
similarity of the patterns in Figure 2-2 A make it clear that the mixed metal systems are 
isostructural with the parent compounds. An expansion of the region between 29 values 
of 16-20° in Figure 2-2B shows the 1 10, 030, and 01 1 reflections for the same three 
compounds. For the mixed-metal example, discrete 1 10 and 01 1 reflections are observed 
at 29 values between those of the pure Mn 2+ or Co 2+ phases, while the 030 reflection 
remains the same for all three samples. These observations are consistent with the 
formation of a single homogeneous solid solution. Similar results were seen for all 
compositions, and Table 2-1 lists the corresponding a, b, c cell parameters for the pure 
and doped materials as calculated from the 1 10, 030, and 01 1 reflections. The cell edge 
lengths systematically shift in value as a function of x. The absence of any reflections 
corresponding to the pure single ion phenylphosphonates in the XRD patterns of the 
mixed-metal phenylphosphonates, combined with the observation that the detected 
reflections have 29 values between those of the two pure compounds, provide convincing 
evidence that single phase solid solutions have been formed. 
Electron Paramagnetic Resonance 

Evidence for microscopic homogeneity of the solid solutions comes from EPR. 
The cobalt phosphonate is EPR silent at X-band, while the manganese analog gives a 



42 




2000 4000 6000 

Magnetic Field (G) 



Figure 2-3. Room temperature EPR signals for polycrystalline a: Co(0 3 PC 6 H 5 )H 2 0,b: 
Mno g 4Coo.i6(0 3 PC6H5)H 2 0, c: Mn(0 3 PC 6 H 5 ) H 2 






broad line that is structureless as a result of dipolar interactions (Figure 2-3). 132 The 
anisotropy of the EPR line width has previously been used to demonstrate the two- 
dimensional exchange pathways in the layered manganese phosphonates. 109 ' 132 As the 
percentage of Co 2+ in the solid solution increases, the Mn 2+ signal broadens (Figure 2-3) 
reflecting the randomization of the identity of the Mn 2+ ions nearest neighbors. In the 
solid solution, there is no signal due to crystallites of pure Mn(0 3 PC 6 H 5 )H 2 0. 
Magnetic Properties of MnfOaPQHg) H 2 and Co(0 3 PC6H 5 ) H 2 

The magnetic properties of several manganese and cobalt organophosphonates 
have been studied previously. 101 ' 102 ' 104 ' 105,108 The manganese phosphonates undergo a 
long-range ordering transition to a canted antiferromagnetic state at temperatures ranging 
from 12 K to 18 K, depending on the identity of the organophosphonate. Pure 
Mn(0 3 PC6H 5 )H 2 orders at T N * 12 K. 108 The cobalt phosphonates also order 
antiferromagnetically, and for Co(0 3 PC 6 H 5 ) H 2 0, we observe T N * 4 K, as we describe 
later in this section. 



43 




50 100 150 200 250 300 

T(K) 




T(K) 



Figure 2-4. A. The temperature dependence of the DC magnetic susceptibility for 
Mn(0 3 PC6H 5 ) H 2 after zero field cooling the specimen to 2 K and then measuring in a 
field of 1 kG. The results of a fit using a S = 5/2 Heisenberg high temperature expansion 
for T > 20 K are shown by the solid line with the result J = -2.27 ± 0.02 K, as described 
in the text. B. The temperature dependence of the DC magnetic susceptibility for 
Co(03PC 6 H 5 ) H 2 after zero-field cooling the sample to 2 K and then measuring in a 
field of 1 kG. The results of a fit using an S= 1/2 Ising high temperature expansion for T 
> 9.5 K are shown by the solid line with the result J = -2.43 ± 0.05 K, as described in the 
text. 



The data in Figure 2-4 show the temperature dependence of the static magnetic 
susceptibility of the pure Mn and Co materials, acquired by cooling the samples in zero 
magnetic field and measuring in a DC field of 1 kG. The broad maximum in the 
susceptibility, Xmax, is characteristic of low dimensional antiferromagnetic interactions 
when short-range order correlations become greater than the thermal fluctuations of the 



44 

spins. These short-range correlations are established by magnetic exchange interactions, 
J, which are typically considered to be limited to nearest neighbor spins. In other words, 
the Hamiltonian may be written as 

3i= -J 2 S,-S, [2-1] 

nn 

where V runs over all pairs of nearest neighbor spins S, and S, The susceptibility data 

nn 

for the manganese phosphonate may be fit with a 2D high temperature series 
expansion for a quadratic layer of Heisenberg S = 5/2 spins based on eq. [2-1], and the 
solid line in Figure 2-4A shows the best fit to the data with J = -2.27 ± 0.02 K. The fit 
was restricted to T > 20 K since at lower temperatures the fitting procedure is not valid. 
In the case of the pure cobalt phenylphosphonate, Eq. [2-1] still describes the simplest 
interactions for the case of this 2D, S = 1/2 Ising system when the spin operators are 
restricted to their z-components. 134 The solid line in Figure 2-4B is a fit, for T > 9.5 K, to 
a 2D, S =1/2 Ising high temperature series expansion, 135 using an exchange constant of J 
= -2.43 ± 0.05 K. It is noteworthy that the magnetic exchange parameters are very 
similar in spite of the significantly different spin values and spin dimension. 

Previous studies 108 have identified the ordering in Mn(0 3 PC6H5)H 2 as a 
transition to a canted antiferromagnetic state in analogy to other manganese 
organophosphonates. The magnetic moments assume a non-collinear orientation that 
produces a weak ferromagnetic moment that lies within the plane of the manganese ions. 
This moment, and hence the transition from the paramagnetic to the canted 
antiferromagnetic state, can be observed in a difference plot of the magnetization as a 






45 



7 

^ 6 

O 
a 5 

£ 



M 

(fc) 



4 - 



3 - <"<•> 



2 - 



^ 4 
O 






2 - 



- 



M 



D 
- 



:D DDDn° 



QdD DL 
J I I I I I I I l__i_ 



A A A Ai 



M 



(fc ifc) 



I ■ ■ ' ' I ' 

5 10 



_i i i i_ 



15 



~r 

20 



T(K) 



Figure 2-5. A. Field cooled (FC) and zero-field cooled (ZFC) magnetization data of 
manganese phenylphosphonate are shown as a function of temperature. Both data sets 
were acquired with a 100 G measuring field. B. The difference between field cooled and 
zero-field cooled magnetization versus temperature for Mn^PCeHs) H2O measured in a 
field of 1 00 G. 



function of temperature for experiments performed in field-cooled (fc) and zero-field- 
cooled (zfc) conditions, AMfc.^ (Figure 2-5). The ordering temperature, T N , may be 
identified in the M k data as the temperature where the magnetization begins to deviate 
from its high temperature paramagnetic behavior, and from Figure 2-5, T N = 1 1.7 K for 
Mn(0 3 PC 6 H 5 ) H 2 0. Another parameter, T N *, is defined as the temperature at which 
AMfc.rft differs significantly from zero. These two temperatures, T N = 1 1.7 K and 



46 





20 - 


□ • Mn Co „ 

0.95 0.05 
□ 




- 


□ Mn,.,Co„ . 

0.82 0.1 S 

D 
D 


~o 
1 
"3 

= 


15 - 


□ 


■? 

e 


10 - 




< 








5 - 


• 






• 






• 










l l l | l l l | l l l | l l l | l l l | l 



8 10 12 

T(K) 



14 



Figure 2-6. The difference between the field cooled and zero field-cooled magnetization, 
AM, is shown as a function of temperature. Typical data from the Mn-rich (i.e. x > 0.25) 
samples are shown when the magnetic field, for measuring and field cooling, was 100 G. 



T N * = 1 1.5 K, are identifiable in Figure 2-5. The value of T N * changes as a function of 
the magnitude of the applied measuring field and as a result, for small values of T N , it is 
best to acquire data with a smaller measuring field, typically 50-100 G. Due to this 
dependence upon measuring field, it is important to realize that T N * will always be lower 
than T N , but T N * is nevertheless evidence of an ordered state with a weak ferromagnetic 
moment. In contrast to the weakly ferromagnetic manganese compound, the pure cobalt 






47 









2.75 - 






. A 

• 


*-s 


• 
• 





• 


= 2.50 - 


\ 


- 


N 


t 
o 


• 
• 
• 


*-* 


• 
• 


s 


• 
• 
• 


2.25 - 


• 
• 
• 
• 
• 
• 




• 
• 
• 
• 








T 


10 - 


N B 


o 






3 




E 
5 




] MHttttyutti 


© 


- 




i - 


1 


-10 - 








i i i | i i i | i i i | i i i | i i i | i i i , i i . | i i i 



2 4 6 8 10 12 14 16 

T(K) 



Figure 2-7. A. Field cooled and zero-field cooled DC magnetization for x = 0. 1 . Both 
data sets were acquired with a 100 G measuring field, and on this scale, the difference 
between the two data sets is not visually detectable. B. The difference between field 
cooled and zero-field cooled magnetization from A is shown as a function of temperature. 
The onset of a negative magnetization occurs at T N , and this signature is characteristic for 
all the Co-rich (i.e. x < 0.25) samples. 



compound is antiferromagnetic with T N = 3.9 K, as determined from both DC 
magnetization and AC susceptibility measurements. 
Magnetic Properties of the Solid Solutions 

Typical magnetization plots for the solid solutions Mn x Coi. x (0 3 PC6H 5 ) H 2 with 
0.25 < x < 1 .00 are shown in Figure 2-6. The ordering temperatures identified in the 
magnetization vs. temperature plots are consistent, within experimental resolution, with 



48 







Figure 2-8. The magnetic phase diagram of MnxCoi.^ChPCeHsyhhO indicating the 
ordering temperature vs. Mn 2+ concentration. The phase diagram has a tetracritical point 
at x = 0.25, as described in the text. The present work was restricted to T > 2 K. The 
lines are guides for the eyes and are described in the text. 



the temperatures of anomalies in the ac susceptibility studies. Like the pure 
Mn(0 3 PC 6 H5) H 2 0, the solid solutions with x > 0.25 form canted antiferromagnets in the 
low temperature state. For x < 0.25, the AMe^a vs. temperature plots still reveal the 
ordering temperature, but the magnitude of AMf C . zfc is much smaller and negative, 
Figure 2-7. This point is discussed further, later in this section. Nonetheless, the 
ordering temperatures were confirmed with AC susceptibility measurements, and they are 
included on Figure 2-8. 



49 

The mixed Mn/Co phenylphosphonates can be thought of as magnetically doped 
pure manganese or pure cobalt lattices with the other metal ion as impurity. 
Consequently, a reduction of Tn from the pure systems is anticipated. Since the magnetic 
exchange interactions and lattices are similar, the primary differences are the spin values 
and the dimension of the spins (i.e., Ising-like or Heisenberg-like). Therefore, the 
reduction of T N is not expected to be as strong as it is for the case of doping with 
diamagnetic spins, and these general tendencies are reflected in the phase diagram in 
Figure 2-8. In the Mn-rich regime, Tn(x) closely follows a linear function with an x = 
intercept (solid line) at the Tn value obtained for the pure Co material. For 0.25 < x < 
0.60, the perturbation of the magnetic correlations is stronger as the percolation threshold 
is approached and the reduction of T N follows a trend qualitatively represented by the 
dotted line. For the Co-rich samples, there is not sufficient resolution in the identification 
of T N to allow a specific x dependence to be identified, so the general trend is sketched 
by the dashed line. The prediction of a tetracritical point at x = 0.25 agrees well with a 
face-centered square planar lattice containing four nearest neighbors where one spin 
species dominates the magnetic exchange. In our case, the Mn 2+ spins dominate the local 
magnetic environment. Tetracritical points 120136 have been observed previously in other 
doped magnetic systems containing competing magnetic anisotropics. 137 
Search for Spin Glass or Precursor Phases 

The assignment of the pure Mn 2 " material as a canted antiferromagnetic S = 5/2 
Heisenberg-like system and the pure Co 2 " system as a quantum antiferromagnetic S = 1/2 
Ising-like system opens the possibility of forming a spin-frustrated state in a randomly 
mixed Mn/Co system. In molecular magnetism, similar studies on layered materials have 
been reported. 1 8139 Thus, bimetallic oxalato layered mixed-metal compounds containing 



50 

competing ferro- and antiferromagnetic interactions have been magnetically characterized 
and in some cases spin glass behavior has been observed. 140 Spin-frustrated systems 
displaying magnetic properties characteristic of spin glasses have also been observed in 
doped magnetic materials possessing tetracritical points in their magnetic phase 
diagrams. 121123141 

Time dependent thermal remnant magnetization studies were performed with two 
samples, x = 0.30 and 0.68. In one set of experiments, the samples were zero-field 
cooled from 300 K to 5, 7, and 12 K in three separate runs. The process of cooling from 
300 K to the low temperature fixed point required approximately 80 minutes. After 
equilibrium was established, a field of 1 kG was applied, and the magnetization was 
monitored for nominally 40 minutes. During this time, the magnetization was observed 
to relax towards an equilibrium value, and this process was easily fit by a simple 
exponential function, yielding time constants ranging from 700 to 1 100 seconds. In a 
different measurement, the magnetization relaxation rates of the sample holders were 
studied and were determined to be negligible. The total change of the signal during the 
measurement after achieving the equilibrium state, as defined by the thermometer of the 
instrument, was about 1%. Although these results may be suggestive of behavior 
associated with a spin glass state, we consider them to be related to the process of cooling 
the powder samples. A simple cooling model 142 provides a plausible explanation for the 
measured relaxation rates. It is noteworthy that the same type of behavior was observed 
for both samples and at all three of the temperatures that were studied. In other words, 
the experiments covered several of the magnetic phases shown in Figure 2-8, and in 
every instance, the behavior was always the same. 



51 



3 
SI 




-r 
5 



10 



t — i — |— r 
15 



T(K) 



Figure 2-9. The real component of the AC susceptibility for Mno.i8Coo.82(03PC6H 5 )H 2 
at 17 Hz and an amplitude of 4 G. The AC susceptibility was studied in an applied field 
of zero and 1 kG, corresponding to the open and filled circles, respectively. The 
identification of T N is consistent with the values determined by DC magnetization 
techniques. The AC response is understood as arising from the dynamics of the magnetic 
domains and the powder nature of the specimens, as described in the text. 



In a second set of studies, the AC susceptibility of samples was investigated. The 
temperature dependences of the real component of the AC susceptibility in applied 
magnetic fields of zero and 1 kG are shown in Figure 2-9 for x = 0.82. Our AC studies of 
all x reproduced, to within experimental resolution, the ordering temperatures seen in the 
DC magnetization data. However, when no external DC magnetic field was present, new 
peaks were observed in the AC susceptibility signals, and these features were not present 
in the DC magnetization data. Upon application of a 1 kG field, these features were 
suppressed and, therefore, can be attributed to the dynamics of the magnetic domains and 
the powder nature of the specimens. The temperatures of the transitions as measured by 



52 

AC susceptibility did not appear to be frequency dependent from 17 Hz to 1.5 kHz. 
Therefore, no spin glass behavior was observed in any of the samples at any temperature 
T>2K. 

In summary, no evidence of a spin glass state was obtained in our measurements. 
It is important to note that a spin-flop transition has been observed in pure 
Mn(0 3 PC 6 H 5 ) H 2 in magnetization vs. field studies performed at 2 K, 108 and a similar 
spin-flop transition is seen for the solid solution with x = 0.84. However, no spin-flop 
signatures were observed for samples with x < 0.84, where an increasing intrinsic 
background arising from competing magnetic spins may have masked the spin-flop 
transitions. Furthermore, we were particularly curious about the possibility of precursor 
behavior in the region near the tetracritical point, i.e. a region bounded by the solid, 
dotted, and broken lines in Figure 2-8. However, as discussed at the beginning of this 
section, no magnetic glassy behavior was observed in this region. Finally, we note that 
for x = 0.30 and 0.35, our studies down to 2 K did not reveal any anomalies indicative of 
crossing into an "intermediate" phase. 120 Naturally, specific heat studies may provide 
additional information concerning the existence of and the identification of such a phase. 
Negative Magnetization in the Cobalt-Rich Samples 

For x < 0.25, ordering is observed, but the value of AMf c .zf C is small and negative, 
Figure 2-7. Features in the ac susceptibility are observed at the same temperatures, so we 
associate these temperatures with the transition to long-range antiferromagnetic order. 
The negative magnetization shifts observed for the Co-rich specimens contrasts with the 
positive magnetization shifts detected for the Mn-rich materials. The phenomenon of 
negative magnetization has been identified previously in a variety of ferrimagnetic 



53 

materials. 127 ' 128 Naturally with the doped Co-rich specimens, similar arguments may be 
made if small regions of ferrimagnetic ordered phase are present. However, negative 
magnetization is also observed in the pure Co material, although it is even weaker than 
observed in the data shown in Figure 2-7B. Negative magnetization has previously been 
observed in the canted antiferromagnet, 143 and the same phenomenon may be responsible 
for the behavior observed for the cobalt phase (x < 0.25). 

Conclusion 
A new series of mixed metal phenylphosphonate solid solutions, 
Mn x Coi.x(03C6H5) H 2 0, have been prepared and their magnetic properties investigated. 
Each composition undergoes long-range magnetic ordering to a canted antiferromagnetic 
state at temperatures, T N $ 12 K, and a magnetic phase diagram has been constructed 
based on individual DC and AC susceptibility measurements. For both the Mn 2+ and 
Co high concentration limits, T N decreases relative to the pure single ion phosphonates, 
consistent with what is expected for magnetic ion impurity doping. The phase diagram 
includes four phases with a tetracritical point at x = 0.25 K, indicating a competition 
between the Heisenberg-like Mn 2+ and the Ising-like Co 2+ spins, with the S = 5/2 Mn 2+ 
dominating the local environment. While prior studies on mixed-metal systems 
possessing competing spin types have shown evidence for spin-glass behavior, no such 
state is observed in the Mn x Coi. x (0 3 C6H 5 ) H 2 solid solutions. 



CHAPTER 3 

STRUCTURE CHARACTERIZATION OF METAL PHOSPHONATE LANGMUIR- 

BLODGETT FILMS BY GRAZING INCIDENT X-RAY DIFFRACTION 



Introduction 

We have recently described the preparation of a series of metal phosphonate 
containing Langmuir-Blodgett flmMMiUWUBUHMJ These films mode led after 
known layered organic/inorganic solids, demonstrate that it is feasible to incorporate an 
inorganic extended solid-state network into the hydrophilic region of a Langmuir- 
Blodgett bilayer assembly. Examples prepared to date include several 
divalent 1 11 - 113114 ' 119132 - 145 and trivalent metal phosphonate networks 113114117 formed with 
straight chain alkylphosphonates as well as with organophosphonate amphiphiles 
containing azobenzene 11 and tetrathiafulvalene 119 functional groups. The inorganic 
lattice greatly enhances the stability of the resulting LB films 118 and enables introduction 
of traditionally solid-state properties such as magnetism into these thin film 
materials. In addition, by employing functional phosphonic acids, LB films that 

combine properties of organic and inorganic assemblies have been prepared. 117 ' 119 

To date, direct structural characterization of the inorganic component of these 
metal phosphonate films is lacking. Analysis has generally relied on comparisons of 
spectroscopic and magnetic properties of the films with those observed for similar solid- 
state analogs. These comparisons have offered convincing, but nevertheless indirect, 
evidence that the inorganic lattices formed in the LB films are isostructural with the 



54 



55 

known solids. We report here direct structural characterization of the inorganic lattices in 
a series of these metal phosphonate containing LB films using grazing incidence X-ray 
diffraction (GIXD). The results confirm the earlier conclusions that the inorganic lattice 
that forms in these LB films is isostructural with the solid-state analogues. 

Experimental Section 

The 16-bilayer manganese octadecylphosphonate 145 (MnOPA), 16-bilayer 
lanthanum octadecylphosphonate 114 (LaOPA), and 15-bilayer manganese (4-(4'- 
tetradecyloxyphenyldiazenyl)phenyl)butylphosphonate 117 (MnA4) LB films on glass 
slides were prepared as previously described. A "bilayer" is comprised of head-to-head 
layers of the phosphonate amphiphile sandwiching one metal ion layer. 

The grazing incident X-ray diffraction experiments were conducted at the 
Materials Research Collaborative Access Team (MRCAT) beamline at sector 10 of the 
Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois. 76 The 
beamline is equipped with an undulater insertion device, a double silicon crystal 
monochromator, harmonic rejection mirror, and a Nal scintillation counter mounted on 
an eight-circle Huber goniometer. 146 The sample was mounted in the center of the 
goniometer and aligned to make the X = 1.254 A X-rays incident on the sample at an 
angle of 1 .8 mrad. The evanescent wave produced at this low incidence angle allows for 
enhanced surface sensitivity. The beam was 2 mm wide by 0.2 mm high and irradiated 
an approximately 50 mm strip along the sample surface. The scattered X-rays were 
detected by scanning the scintillation counter through a plane parallel to the sample 
surface. Bragg rod profiles were obtained by scanning over the peaks of interest in the xy 
plane with successive steps in the z direction (perpendicular to the sample surface). The 



56 





Figure 3-1 In-plane and cross-sectional view of Mn(0 3 PC6H5)H 2 0. Crystallographic 
data are taken from reference 16. Key: oxygen, small open circles; manganese, 
crosshatched circles; phosphorus, diagonal-hatched circles (phosphorus atoms above and 
below the plane are distinguished by hatches with different directions). 



scattered X-rays were collimated through a set of Soller slits prior to detection giving an 
instrumental resolution in Q XT of 0.0015 A' 1 , and Q z of 0.01 A" 1 where 
Qxy= (47c/a.)sin9xy and Q z = (27r/\)sine z . 

Results and Discussion 
Manganese Octadecylphosphonate Film 

The structural prototype of the Mn(0 3 PR) H 2 manganese phosphonates is the 
phenyl analog, Mn(03PC 6 H5)H 2 0, the structure of which was determined by Cao et al 
and is shown in Figure 3-1. 91 The solids crystallize in the Vrrm2 x space group, and the 
phenylphosphonate has unit cell parameters a = 5.734, b = 14.33, and c = 4.945. The 
lamellar material consists of two-dimensional manganese-oxygen networks in the ac 
plane separated in the b direction by the phenyl groups of the phenylphosphonate ligands. 
If the in-plane manganese-oxygen network is considered alone, it can be described with a 



57 






& 

s 
o 
U 




5.5 5.0 4.5 4.0 3.5 3.0 2.5 

d,(A) 



Figure 2. The grazing incidence X-ray diffraction pattern obtained on a 16-bilayer 
sample of MnOPA. 



face-centered rectangle unit cell with edges a = 5.734 A and b = 4.945 A (in the two- 
dimensional cell, the b axis corresponds to the c axis in the P/w/2 f space group). 

The background corrected GIXD pattern obtained from the MnOPA LB film is 
shown in Figure 3-2. The pattern consists of four distinct diffraction peaks corresponding 
to lattice spacings of 4.27, 3.71, 2.88, and 2.44 A. The latter three peaks are due to 
scattering from the manganese-oxygen network of the inorganic lattice and can be 
indexed to a face-centered rectangular cell with Miller indices of (1 1), (2 0), and (0 2), 
respectively, corresponding to unit cell parameters of a = 5.76 A and b = 4.88 A. The 
(1 0) and (0 1) reflections are absent due to the centered cell. The indexed diffraction 
pattern of the LB film agrees very well with the unit cell for the solid-state manganese 
phenylphosphonate verifying that the inorganic network formed in the LB film is 
isostructural with the analogous solids. Analysis of the peak width for the isolated 



58 

(0 2) peak by application of the Scherrer equation 147 yields an average structural 
coherence length of -180 A. 

The intense broad peak in the diffraction pattern at a spacing of 4.27 A is typical 
of inter-alkyl chain distances. The presence of a single diffraction peak is usually 
interpreted as hexagonal close packing of freestanding chains. This packing motif is 
unlikely in MnOPA due to the structural constraints of the underlying inorganic network 
that yields an average cross-sectional area per alkyl chain of 28 A 2 . Interlayer spacings 
determined from previous X-ray diffraction data indicate a tilt angle of approximately 
30°, and Bragg rod scans of the GIXD peak confirm this assessment. The absence of the 
additional diffraction peaks that are expected from a lower symmetry cell is likely due to 
a small coherence length of the alkyl packing, resulting from significant disorder in the 
organic network induced by the lattice mismatch between the rigid inorganic layer and 
the alkyl chain packing. A similar situation has been observed for alkyl chains tethered 
to a rigid polymer backbone. 148 
Azobenzene Derivatized Manganese Phosphonate Film 

The GIXD pattern for a 1 5-bilayer film of MnA4 is shown in Figure 3-2. As in 
MnOPA, the pattern shows the expected peaks corresponding to the (2 0) and (0 2) 
Bragg planes for the manganese phosphonate lattice at spacings of 2.88 A and 2.44 A. 
The (11) peak expected at 3.71 A is obscured in MnA4 by the strong scattering from the 
organic groups. The spacings for the inorganic lattice yield the same centered 
rectangular cell, a = 5.76 A and b = 4.88 A, observed for the MnOPA film, verifying that 
the same inorganic network forms in each case. 



59 




iiil|ltli|iiii|iiii|ifii|itii| 

5.5 5.0 4.5 4.0 3.5 3.0 2.5 



d,(A) 



Figure 3-3. The grazing incidence X-ray diffraction pattern obtained on a 15-bilayer 
sample of MnA4. 



As expected, the packing of the organic groups is different in the two manganese 
phosphonate films. In contrast to the intense reflection at 4.27 A in MnOPA, the MnA4 
film has a strong reflection at 3.79 A and a weaker reflection at 4.59 A. Previous 
investigations into the structure of azobenzene-containing alkane thiol monolayers on 
gold by Caldwell et al. suggested that the azobenzene groups, when tethered to the 
surface with flexible alkyl segments, can aggregate into tightly packed islands forming a 
herringbone motif. Thus, the azobenzene unit, although not completely independent of 
the Au(l 1 1) surface, strongly influences the overall packing arrangement. 94 In the case of 
the SAMs on gold, the azobenzene groups were proposed to pack with hexagonal 
symmetry with an intermolecular separation of 4.5 A, which gave rise to a single 
diffraction peak at 3.9 A. The presence of two diffraction peaks at 4.59 A and 3.79 A in 
MnA4 suggests a deviation from hexagonal symmetry and therefore a different packing 



60 

arrangement of the azobenzene groups than was observed for the SAMs on gold. This 
variation likely results from a combination of the rectangular symmetry of the underlying 
manganese phosphonate lattice and the shorter alkyl tether in MnA4 preventing a high 
degree of aggregation. A unit cell for the organic network can be assigned by assuming 
the azobenzenes tilt by 22° along the direction parallel to the manganese face diagonal. 
This arrangement would result in an oblique unit cell of dimensions a = 7.0 A and b = 5.8 
A and y = 140° for which the reflections at 4.59 A, and 3.79 A can then be assigned to the 
(1 0) org and (0 l)or g Bragg planes, respectively (the subscript org is used to differentiate 
the organic network when it is considered independently from the inorganic network). 
The (2 -2) org reflection for the organic lattice is predicted at 2.88 A, commensurate with 
the (2 0) reflection of the parent inorganic network. This peak may contain intensity 
from both networks. Support for this assignment comes from the peak width, which is 
larger than either the related 2.44 A peak that arises from the inorganic network or the 
corresponding peak from Figure 3-1 for MnOPA. This arrangement of the azobenzene 
groups would also lead to significant ^-interactions perpendicular to the (0 l) OT g Bragg 
plane and 7t-stacking is supported by UV-Vis spectroscopy, reported previously." 7 
Lanthanum Octadecylphosphonate Film 

Trivalent lanthanum phosphonates are also known to form layered structures in 
the solid state, now consisting of two-dimensional oxygen-bridged La 3+ networks 
separated by the organic substituents of the phosphonate group. The GIXD pattern for 
the LaOPA LB film is compared to the diffraction pattern obtained for a powdered 
sample of lanthanum butylphosphonate in Figure 3-4. The similarities in the diffraction 
patterns of the two materials indicate that the inorganic network formed in the LB film 



61 



e 

8 

u 




d,(A) 



Figure 3-4. The grazing incidence X-ray diffraction pattern obtained on (top) a 16- 
bilayer sample of LaOPA compared to a X-ray powder diffraction pattern of (bottom) 
lanthanum butylphosphonate. 



is isostructural with the shorter chain solid-state analog. The trivalent lanthanum ion 
requires two phosphonates (one monobasic and one dibasic) to preserve electric 
neutrality, resulting in a higher density of alkyl chain packing than is observed for the 
manganese phosphonates. Bragg rod scans of the GIXD peaks and interlayer spacings 
obtained from conventional X-ray diffraction indicate an absence of any significant tilt 
angle in the alkyl chains. As a result, the organic and inorganic networks lie on the same 
two-dimensional lattice, and the observed diffraction peaks are a composite of scattering 
from the inorganic extended network and the alkyl chain molecular network. The entire 
pattern can be indexed to an oblique cell with a = 12.05 A, b = 10.55 A, and y = 72°. The 
assigned unit cell is essentially a 1.5*a, 2*b super cell of the in-plane cell derived from 
X-ray powder diffraction studies of lanthanum methylphosphonate. 149 More complete 
structures of lanthanum benzylphosphonate and lanthanum phenylphosphonate 









62 



Table 1 . The calculated and observed lattice (d) spacings, in A, for the proposed 
lanthanum octadecylphosphonate unit cell, a = 12.05 A, b = 10.55 A, and y = 72°. 






(h,k) 


calc 


obs 


(0,2) 


5.02 


5.0 


(2,2) 


4.53 


4.57 


(3,0) 


3.82 


3.85 


(3,2) 


3.63 


3.68 


(3,-2) 


2.67 


2.67 


(2,-3) 


2.56 


2.56 


(-2,4) 


2.07 


2.11 


(4,-3) 


1.90 


1.98 



have been determined; 150 however, the methyl derivative was chosen as the structural 
analogue to the LB film since both are alkylphosphonates. The methylphosphonate 
diffracts in a triclinic space group with a = 5.398, b = 8. 168, c = 10. 162, a = 73.76°, 
3 = 83.89°, and y = 73.5°. 149 The in-plane lanthanum-oxygen network of an individual 
layer can be assigned two-dimensional parameters based on this structure of a = 5.398 A, 
b = 8. 168 A, y = 73.5° with two lanthanum ions per unit cell. The larger unit cell for the 
LB film is necessary to make the organic and inorganic sublattices commensurate. The 
calculated lattice spacings are compared to the experimental spacings in Table 3-1 . This 
cell contains three lanthanum ions and six phosphonate groups and provides an average 
molecular cross-sectional area per phosphonate group of approximately 20 A 2 , consistent 
with a close-packed, upright organization of the alkyl chains. 



63 

Conclusions 
The GIXD experiments on a series of Mn 2+ and La 3+ metal phosphonate LB films 
prove that the inorganic networks in these films are isostructural with their known solid- 
state analogs, confirming earlier assignments that were based on spectroscopic data. The 
three examples described here make up an interesting series. The LaODP film provides 
an example where the inorganic and organic networks can be described with the same 
two-dimensional unit cells. In the MnA4 film, the two networks are commensurate, but 
best described with independent cells. And finally, the packing of the organic network in 
the MnODP film appears to be incommensurate with the inorganic network. These 
observations reinforce the idea that the metal phosphonate extended lattice will form 
regardless of the organic groups, as long as the space requirements of the metal ion lattice 
can be met. The larger energy associated with the metal/ligand interactions determines 
the structure and the area available to the organic groups. 



CHAPTER 4 

FORMATION OF AN EXTENDED TWO-DIMENSIONAL COORDINATE 

COVALENT SQUARE GRID NETWORK AT THE AIR WATER INTERFACE 



Introduction 

Many advances in the pursuit of nanoscale objects make use of supermolecular 
assembly, the synthesis of larger structures from molecular building blocks. 1 Inspired by 
biological self-assembly, much supermolecular chemistry holds structures together with 
directed, non-covalent interactions such as hydrogen bonding, van der Waals, 
electrostatic, and 7t-stacking forces. 2 ' 4 However, bonding is not restricted to weak 
interactions, and the directional properties of coordinate covalent bonding have also lead 
to many interesting structures. 2,4 " 13 

Among the motivations for the pursuit of nanometer scale objects is the need 
for electronics architectures that are beyond the scope of present-day lithographic 
technologies. ' Such architectures will require both nanometer scale device 
components and infrastructures such as wires, insulation, and shielding that can service or 
interface with these devices at the nanometer scale. 2 In addition to electronics and 
information storage, other applications of nanoscale architectures include catalysis and 
separations, while nanoscale objects also have tremendous potential as molecular level 
probes and transducers for chemical recognition sensing. 152 

Many of these applications are likely to require positioning the structures at 
surfaces. For example, the electronics architectures mentioned above will have to be 



64 



65 

fabricated onto a support. Two-dimensional (2D) grid structures have been proposed as 
separations media, 153 " 156 , which will require their positioning at an interface between 
phases. Interfaces may also play a role in the "manufacture" of supermolecular 
structures, providing a way of directing interactions by orienting molecules at the surface 
for subsequent reaction. Therefore, there is a significant need to investigate the 
application of supermolecular assembly processes at interfaces. With a growing 
understanding of how to synthesize supermolecular objects, we can now begin to study 
how the requirements of such assembly processes can be adapted for fabrication at 
interfaces. Included in this goal is the need to investigate ways to use the interface itself 
as a structure-directing feature in the assembly of supermolecular architectures. 

Air/liquid interfaces are often used to direct assembly processes, and a careful 
understanding of these processes is now possible largely as a result of surface sensitive 
characterization methods, including grazing incidence X-ray diffraction. Traditional 

f\f\ 70 RO 1 S7 

Langmuir monolayers can form two-dimensional molecular crystals, ' or can 

selectively bind molecules or ions from the subphase to produce multicomponent 
assemblies. 80 " 158 " 160 Langmuir monolayers are also used to induce the heterogeneous 
nucleation of three-dimensional crystals, where chemical or stereochemical features of 
the monolayer can direct the morphology, orientation, or chemical identity of the product 
crystals. 161 " 163 Supermolecular objects have also been prepared in situ at the air/water 
interface. 80 For example, the 2x2 and 3x3 metal ion molecular grids first described by 
Lehn et a l. 164 " 166 been formed at the air/water interface by reaction of Langmuir 
monolayers of the linear multidentate ligands with aqueous metal ions. 62 " 63 These and 
other examples are included in a recent comprehensive review by Kuzmenko et al. 80 



66 

Less common are extended two-dimensional covalent grid networks. The best-described 
examples are those of Michl and coworkers who use the principles of modular chemistry 
to prepare surface-anchored two-dimensional covalent networks. 153 " 155 ' 167 ' 168 

As part of our investigation of these issues, this article describes the assembly of a 
two-dimensional nickel-iron-cyanide grid network at the air/water interface. Numerous 
solid-state compounds based on bridging cyanides are ^own 19 - 21 ' 22 ' 33 - 38 - 51 ' 52 ' 169 - 172 w j t h 
the prototype being Prussian blue. Recent interest in these compounds stems from the 
cyanide ligand's ability to efficiently mediate magnetic exchange, and many new mixed- 
metal Prussian blue-like structures have been developed with fascinating magnetic 
properties. ' ' Structures with one- and two-dimensional coordinate covalent 

networks are also known, 33170171 and metal cyanide complexes have been used as 
building blocks in the preparation of "zero-dimensional" clusters. 19 ' 21 ' 22 ' 172 In all cases, 
the structure directing elements are the well-defined bond angles of the transition metal 
complexes and the linear bridging cyanide ligands. 

Our approach for assembly at an interface is outlined in Scheme 4-1. The target is 
a square grid nickel-iron-cyanide network that arises from the 90° bond angles around the 
starting iron cyanide complex. The product is a single monolayer of a two-dimensional 
square grid because the amphiphilic dialkylaminopyridine ligand confines the iron 
complex to the interface, which then directs the condensation reaction within the plane of 
the water surface. In the absence of the interface, the pentacyanoferrate (3+) starting 
complex is capable of forming bridges that lead to geometries other than a square grid, 
and when the reaction is carried out in solution, only amorphous products are observed. 









67 




R = (CH 2 ) 11 CH 3 










R^ ,R 

N 




N N" 



Ni(H 2 0) 6 (N03) H ^n 

► N 




I 

Fe- TT -Ni- 



N C I C ^im subphase Fe - N Ni<— ^Fe — 

N C ^-^ l.^-"" ^^ I 

III — Ni Z^Fe~ — Ni — 

" III 

Langmuir monolayer 2D grid network 



Scheme 4-1. Assembly of a two-dimensional square grid network at the air/water 
interface. 



The interface facilitates bridging in the equatorial plane of the amphiphilic complex, and 
therefore plays an important role in controlling the final structure. 

A potential obstacle to confining reactants to an interface is that reactivity can be 
limited by restricted diffusion. 156 A gas/liquid interface minimizes this problem, allowing 
studies to focus on the structure-directing elements of the reactants and surface. In 
addition, structures formed at the air/water interface can be transferred from the water 
surface to solid supports using standard Langmuir-Blodgett film methods. The 
transferred films allow for a more thorough measurement of the structural and physical 
properties of the interface-formed networks. The condensation reaction outlined in 
Scheme 4-1 is followed at the air/water interface with surface pressure measurements and 
with Brewster angle microscopy (BAM), and the structure of the resulting nickel-iron- 
cyanide network is confirmed in transferred films with optical and infrared spectroscopy, 
X-ray absorption fine structure (XAFS), grazing incidence X-ray diffraction (GIXD) and 
magnetization measurements. 



68 

Experimental Section 
Synthesis 

Materials. Unless otherwise indicated, all reagents were purchased from Aldrich 
(Milwaukee, WI) or Fisher Scientific (Pittsburgh, PA) and used without further 
purification. The 4-aminopyridine was recrystallized from water prior to use. 

Instrumentation. All NMR spectra were obtained on a Varian VXR-300 
spectrometer. The characteristic solvent peaks were used as reference values. Elemental 
analyses and mass spectrometry analyses were performed by the University of Florida 
Spectroscopic Services laboratory, where high-resolution mass spectra were collected on 
a MAT 95Q, Finnigan MAT (San Jose, CA). Melting points were obtained on a Thomas- 
Hoover Capillary melting point apparatus and are uncorrected. UV-Vis spectra were 
obtained on a Hewlett-Packard 8452A diode array spectrophotometer. IR spectra as KBr 
pellets were recorded on a Mattson Instruments (Madison, WI) Research Series- 1 FTIR 
spectrometer with a deuterated triglycine sulfate (DTGS) detector. 

N-methyl-4-didodecylaminopyridinium iodide (1). A solution of 3.54 g (0 015 
mol) N-methyl-4-aminopyridinium iodide 175 and 9.37 g (0.0375 mol) of 1- 
bromododecane in acetonitrile (75 mL) was refluxed over 5.5 g K 2 C0 3 (0.04 mol) for 
three days. The acetonitrile was removed and the organic materials dissolved in 
chloroform and filtered. The chloroform was removed under reduced pressure and 20 
mL of diethyl ether was added to the orange oil that remained. Addition of the ether 
solution to 150 mL of pentane with vigorous stirring precipitated the product. The solid 
was filtered and washed well with diethyl ether and dried under vacuum (7.8 g, 91%) 'H 
NMR (CD 3 C1), ppm: 8.45, d, 2H; 6.79, d, 2H; 4.11, s, 3H; 3.37, t, 4H; 1.54, m, 4H; 1.19- 



69 

1.25, m, 36H; 0.80, t, 6H. Calcd for C30H57N2I: C, 62.92; H, 10.03; N, 4.89. Found: C, 
63.25; H, 10.64; N, 4.89. Melting point: 99-101 C. MS (445). 

4-didodecylaminopyridine (2). Demethylation of 1 was accomplished in a 
manner analogous to a previously reported procedure for the demethylation of pyridinium 
salts. A stirred mixture of 7.5 g of 1 and 40 g of pyridine hydrochloride were refluxed 
under nitrogen in the absence of solvent. After 24 hours the mixture was cooled and 75 
mL of water was added to dissolve the excess pyridine hydrochloride. The crude product 
was filtered off and redissolved in 100 mL chloroform. The chloroform solution was 
extracted three times with 50 mL portions of concentrated ammonium hydroxide and 
dried over anhydrous MgS0 4 before removal of the solvent under reduced pressure. 
Acetonitrile (100 mL) was added to the oil that remained and the mixture was vigorously 
stirred in an ice bath. The precipitated solid was redisolved in diethyl ether (100 mL), 
treated with 200 mg of activated carbon and filtered through Celite. The ether filtrate 
was mixed with 80 mL of acetonitrile and concentrated under a stream of N 2 to 
precipitate the pure product as beige solid. The solid was washed with acetonitrile and 
dried under vacuum (3.9 g, 70%). ! H NMR (CD 3 C1), ppm: 8. 16, d, 2H; 6.41, d, 2H; 3.25, 
t, 4H; 1.57, m, 4H; 1.26-1.31, m, 36H; 0.88, t, 6H.. Calcd for C 2 9H 5 4N 2 : C, 80.86; H, 
12.64; N, 6.50. Found: C, 81.14; H, 12.28; N, 6.58. mp 54-56 C MS (431 (+FT)). 

Bis(tetramethylammonioum) pentacyano(4-didodecylaminopyridine)- 
ferrate(m) ' 6H 2 (3). The preparation of the amphiphilic pentacyanoferrate complex 
was adapted from a previously reported procedure for the preparation of disodium 
pentacyano(4-octadecylamino-pyridine)ferrate(III). 177 To a solution of 1.9 g (0.0044 mol) 
of 2 in methanol (50 mL) at 40 °C was added 0.40 g (0.0015 mol) 



70 

Na 3 [Fe(CN) 5 NH 3 ] xH 2 0. The suspension was stirred for 12 hours in air yielding a dark 
purple solution. The methanol was concentrated at room temperature under reduced 
pressure to a volume of 10 mL and 40 mL of chloroform added. The insoluble iron salts 
were filtered off through Celite and the solvents removed. The product was dissolved in 
50 mL methanol and precipitated by the addition of AgBF 4 (0.003 mol) in 25 mL 
methanol. The solid was filtered, washed with methanol and ether, and transferred to a 
methanol solution of tetramethyl ammonium bromide (0.003 mol). The mixture was 
stirred vigorously for four hours and then filtered to remove the AgBr. The violet filtrate 
was concentrated at room temperature to a few milliliters and added to 75 mL 
acetonitrile. Concentration of the acetonitrile under a stream of nitrogen, followed by the 
addition of several volumes of acetone precipitated the complex as a violet powder, 
which turns blue upon hydration. The solid was dried under vacuum in a desiccator over 
P 2 5 (0.412 g, 35 %). IR (KBr pellet): Vc-N^m 1 ) 2126, 2116. Calcd for C42H 9 oN 9 06Fe: 
C, 57.78; H, 10.4; N, 14.44. Found: C, 57.30; H, 10.76; N, 14.68. 
Films 

Materials. Unless noted, all reagents were used as received. 

Substrate preparation. Single-crystal (100) silicon wafers, purchased from 
Semiconductor Processing Co. (Boston, MA), were used as deposition substrates for X- 
ray photoelectron spectroscopy (XPS). X-ray diffraction, FT-IR, UV-Vis, and GIXD 
samples were prepared on petrographic slides that were purchased from Buehler Ltd 
(Lake Bluff, IL). Samples for SQUID and XAFS investigations were prepared on Mylar 
(Dupont) substrates cleaned prior to use with absolute ethanol. The silicon, glass, and 
quartz substrates were cleaned using the RCA procedure 178 and dried under nitrogen. 



71 
All substrate surfaces were made hydrophobic by deposition of a monolayer of 

QJO 179,180 

Instrumentation. The LB films were prepared by using a KSV Instruments 5000 
trough modified to operate with double barriers. The surface pressure was measured with 
a filter paper Wilhelmy plate suspended from a KSV microbalance. Subphase solutions 
were prepared from 17.8-1 8. 1 MQ cm water delivered with a Barnstead Epure system. 
The XPS spectra were obtained on a Perkin-Elmer (Eden Prairie, MN) PHI 5000 series 
spectrometer using the Mg Ka line source at 1253.6 eV. Typical operating pressure was 
4 x 10" 10 bar. X-ray diffraction was performed with a Philips APD 3720 X-ray powder 
diffractometer with the Cu Ka line, X= 1 .54 A. Magnetization measurements were 
performed on a Quantum Design MPMS SQUID magnetometer. 

GIXD and XAFS experiments using synchrotron radiation were performed at the 
Advanced Photon Source, Argonne, IL, at the Materials Research Collaborative Access 
Team beamline (sector 10). The XAFS spectra of LB films transferred to Mylar were 
recorded in fluorescence mode using a Lytle detector. The sample film was oriented at 
45 degrees to the incident beam and the detector at 90 degrees relative to the incident 
beam. Energy calibration was accomplished by simultaneously recording transmission 
XAFS spectra through the appropriate metal foil positioned behind the LB film sample. 
Transmission intensity was measured using an ion chamber detector charged with the 
appropriate air/N 2 ratio to give a linear response over the scanned energy range. The 
sample used for XAFS was 100 bilayers. XAFS scans were taken over both the Fe and 
Ni edges from 150 eV before the edge step to 1 keV beyond the edge in separate runs for 
each edge. Data analysis was performed using the Winxas program. 84 Background 









72 

subtraction and normalization to the edge step was done with a linear fit to the pre-edge 
region and a second order fit to the post edge region. The atomic absorption correction 
was done on the k weighted data using a cubic spline function. Fourier transforms of the 
k -weighted data were done in combination with a Bessel window function. No 
smoothing functions or Fourier filters were applied to the data (for an overview of the 
data analysis see Chapter 1 ) The GIXD scans were performed on LB films transferred to 
glass slides. The sample was positioned in the center of an 8-circle Huber goniometer 
and oriented at an angle of 0. 13 degrees relative to the incident beam. The incident 
beam was collimated to 200 microns high by 1 500 microns wide and tuned to a 
wavelength of 1 .254 A. Diffracted intensity in the xy plane was measured using a Nal 
scintillation counter mounted on the Huber goniometer. The diffracted signal was 
collimated prior to the detector using Soller slits giving an experimental resolution on the 
order of 0.015 A" 1 . 

Film preparation. The amphiphilic iron complex 3 was spread onto the water 
surface from a chloroform solution. All multilayer films of the nickel-iron-cyanide 
network were transferred as Y-type films onto hydrophobic substrates at a surface 
pressure of 25 mN/m over a subphase 1 g/L in Ni(N0 3 ) at ambient temperature. The 
average transfer ratios for both the upstroke and downstroke were 0.85. 

Results 
Langmuir Monolayers and LB Film Transfer 

The amphiphilic iron complex 3 forms a well-behaved monolayer on the water 

surface. Brewster angle microscopy indicates the amphiphile is in a liquid expanded state 

at zero surface pressure at room temperature. Compression of the monolayer at room 

temperature produces the surface pressure versus area isotherm shown in Figure 4-1. 



73 



S 
z 

e, 

■~ 
- 

3 
en 

SO 

V 

fa 
2- 
V 

w 

fa" 
•- 

3 
1/5 



SO 
40 




30 

20 


• o 

• o 

• o 

• 


10 


• ° 

\ V 


U 





25 50 75 100 125 

Mean Molecular Area (A ) 



Figure 4-1. Room temperature surface pressure vs. mean molecular area isotherms for 
complex 3 over pure water (open circles) and over a 1 g/L Ni(N03)2 (filled circles). 



The complex is slightly soluble on pure water and a creep of approximately 15 
A 2 /molecule/hour is seen at a surface pressure of 1 mN/m, Figure 4-2. 

The behavior of 3 over a subphase containing Ni 2+ is markedly different from that 
over pure water. Brewster angle microscopy indicates that the monolayer is in a 
condensed phase at zero surface pressure when the subphase contains Ni 2+ . Evidence for 
a condensed phase at zero surface pressure is also give by the isotherm shown in 
Figure 4-1. The mean molecular area of 52 A 2 at the onset of pressure for the complex 
over a Ni + subphase is nearly identical to the mean molecular area at collapse of the 
complex over pure water. Additionally, the slope of the isotherm is steeper when 3 is 
compressed over a Ni 2+ subphase. As will be demonstrated, this behavior results from a 
condensation reaction between the ferric amphiphile and the aqueous nickel ions. 



74 



- 

- 
< 

- 

a 

w 

.2 

"o 

e 
ce 




10 20 30 40 

Time (min) 



Figure 4-2. The change in mean molecular area versus time at a surface pressure (p) of 
lmN/m for complex 3 over pure water (open circles), and over pure water with 
subsequent injection of a Ni(N03)2 solution (at the time indicated by the arrow) under 
the monolayer (filled circles). Condensation of the monolayer occurs immediately after 
injection of the Ni2+ solution. 



The reaction at the air/water interface was also detected by monitoring the change 
in mean molecular area (MMA) versus time at a constant surface pressure as shown in 
Figure 4-2. A monolayer of 3 was first compressed over pure water to a pressure of 1 
mN/m. A slow and linear creep is seen in the film due to the slight solubility of 3. When 
10 mL of a solution of Ni(N0 3 )2 6H 2 (at a concentration to give a final subphase 
concentration of ImM Ni 2+ ) is then injected under the monolayer, a rapid drop in the 
MMA is seen. The MMA eventually stabilizes at a value of 50 A 2 , which after correction 
for the initial creep of ca. 2-3 A 2 , agrees well with the 52 A 2 seen at 1 mN/m in the 
compression isotherm performed over Ni 2+ . shown in Figure 4-2. 

To further characterize the product of the condensation reaction occurring on the 
water surface, the networks were transferred to various supports by the Langmuir- 



75 

Blodgett (LB) technique. The nickel-iron-cyanide network transferred well as Y-type 
films from a subphase containing 1 g/L Ni(N03) 2 at a surface pressure of 25 mN/m 
giving an average transfer ratio of 85% (due to the rigid nature of the film) on both the up 
strokes and the down strokes on all substrates used. In contrast, in the absence of Ni 2+ , 
amphiphile 3 transfers on the down stroke, but washes off on the up stroke. 
Spectroscopic Analyses 

Pentacyanoferrate(III) complexes coordinated to a 4-aminopyridine ligand display 
an intense ligand to metal charge transfer band between 500 and 700 nm, depending on 
the identity of the 4-aminopyridine derivative and the nature of the solvent. 181 " 182 This 
charge transfer band is also observed in the transferred films containing the nickel-iron- 
cyanide network. The intensity of the charge transfer band increases linearly with the 
number of transferred bilayer and demonstrates a reproducible transfer of the network 
from one bilayer to the next. In addition, the presence of this band in the LB films 
confirms that the integrity of the iron complex is preserved over the course of film 
formation. 

Evidence that nickel is incorporated in the transferred film is found by XPS. A 
survey scan of a monolayer transferred on the upstroke onto a clean silicon wafer shows 
both Fe (2pi and 2p 3 ) and Ni (2pi and 2p 3 ) peaks. Analysis of the integrated areas of the 
XPS multiplex scans using a take-off angle of 80 degrees and taking into account 
differences in photoelectron escape depths 183 " 185 for both sets of peaks yields an Fe:Ni 
ratio of 1 : 1 +/- 10%. This ratio is expected if the Ni 2+ ions are incorporated into a face- 
centered square grid assembly as depicted in Scheme 4-1. 



76 



s 

3 

- 
- 

U 

e 
- 

- 
o 



. 


1 — 1— 1 — 1 




-1 — 1 — 1 — 1 — 1— I — 






A 


• 


-— -v^ 


_y 


V 


(a) 


A 




- 




A_ 


(b) ; 


. 




v . 


■ 




j i i i 




. . ■ . . . 



2300 



2200 



2100 



2000 



1900 



Wavcnumber (cm ) 



Figure 4-3. FT-IR absorption spectra of the C-N stretching region for (a) a monolayer 
film of the nickel-iron-cyanide grid network transferred to a silicon ATR crystal, and (b) 
a KBr pellet of pure 3. 



Confirmation of cyanide bridging in the monolayer networks is found by 
comparing the C-N IR stretches in the LB film and a KBr pellet of pure 3 (Figure 4-3). 
In 3, the cyanide-stretching region shows a strong band at 21 1 1 cm" 1 and a shoulder at 
2126 cm" 1 . These bands are in agreement with the pseudo Ctv symmetry of the complex. 
The FT-IR spectrum of a monolayer of 3 reacted with Ni 2+ and transferred to a silicon 
ATR crystal shows a dominant cyanide stretching band at 2162 cm" 1 and a weaker band 
at 2 1 1 8 cm" 1 . This shift to higher energy is typical when cyanide assumes a bridging 
mode. 71 " 73 












77 



s 

.o 
- 
- 

■a 

3 

— 

"2 

es 



- 



i i I 



I I ' T™ 




Figure 4-4. Magnitude of the Fourier transforms (FT) of the (a) nickel edge and (b) iron 
edge k 3 -weighted XAFS for a 100-bilayer sample of the nickel-iron-cyanide grid network 
transferred to Mylar. The "R-axis" has not been corrected for phase shifts. No 
amplification factors were applied to either trace. 



XAFS Analysis 

The non-phase-shift corrected Fourier transforms of the k 3 weighted XAFS of the 
nickel-iron-cyanide network transferred onto Mylar are shown in Figure 4-4 for the iron 
and nickel edges. Both sets of data show a similar pattern with three dominant peaks 
attributed to the first three coordination shells. For the iron edge, the first two peaks 
correspond to the C and N of the cyanide ligand, respectively, and the third peak to the 
nickel ion coordinated to the nitrogen end of the cyanide bridge. For the nickel edge 
transform, the peak assignments can be made with the first peak corresponding to the 
cyanide nitrogen and most likely coordinated water, the second peak to the cyanide 
carbon, and the third peak to the iron atom. The significant intensity of the peaks at 



78 



y— S U.1U 




1 "T 1 1 1- 




n 








•** 


■ 




, 










e 








3 






■ 


• 

■e 


, 




, 


k 








es 








"— ' 








.§ 0.05 


■ 






2 






(B 


a 






¥> 


es 








5 


i# 




V) 


H 






v. ^^^ 


ta 








0.00 






■ 


c 


1 1 


2 
R(A) 


3 4 



Figure 4-5. Fit (solid line) to the first two coordination shells of the Fourier transformed 
nickel edge XAFS (circles) of the nickel-iron-cyanide grid network on Mylar. The fit 
was calculated using FEFF7 codes for a model Ni 2+ cluster coordinated in-plane by the 
nitrogen end of four cyanides and axially by two oxygen ligands. 



approximately 4.5 A in both radial plots has been explained in other cyanide-bridged 
systems as resulting from the focusing effect of the linear cyanide bridge. 186 ' 187 

The results of a fit to the first two coordination shells in the nickel edge k 3 
weighted Fourier transformed XAFS data are shown in Figure 4-5. The fit was 
accomplished using the program Winxas with inputs from theoretical XAFS parameters 
generated from FEFF 7.0 codes 85,86 for a model nickel cluster composed of two axial 
oxygen atoms and four equatorial nitrogen-bound, iron-terminated cyanide ligands. The 
coordination number for Ni was fixed and both shells were fit simultaneously using an 
intrinsic reduction factor (S 2 ) of 0.52 for each and an edge energy shift (AE ) of 1.0 and 
6.0 for the first and second shells, respectively. The S 2 and AE values used in the fit 
were similar in magnitude to those reported in the XAFS analysis of a similar metal- 



79 



cyanide system. 188 The bond distances extracted from the fitting procedure were (in A): 
Ni-N, 2.09; Ni-O, 2.1 1; Ni-C, 3.22; and C-N, 1.13; and are reasonable if compared to 
similar compounds. 37 - 3840 Fits to the Ni edge XAFS were limited to the first two 
coordination shells due to complexities arising from the large number of multiple 
scattering pathways contributing to the third coordination shell. 
X-ray Diffraction and GLXD 

The lamellar order in the multilayer films of the nickel-iron-cyanide network was 
confirmed by X-ray diffraction from a 30-bilayer sample. An intense diffraction peak at 
2.5 degrees 26 and a weaker harmonic at 5 degrees 29 can be assigned to the (001) and 
(002) Bragg reflections and yield an inter-bilayer spacing of 35 A. This inter-bilayer 
spacing is reasonable for the size of the amphiphile deposited as Y-type bilayers. 

Grazing incidence X-ray diffraction was used to verify the presence of any long- 
range in-plane structural correlations in the film. The diffraction pattern obtained for a 
39-bilayer sample of the iron-cyanide-nickel network transferred to glass is shown in 
Figure 4-6. The counts are normalized to the most intense peak and plotted versus the in- 
plane scattering vector Q^ = (47i/>.)(sin9 X v). The three intense peaks can be assigned to 
the (2,0), (2,2), and (4,0) Bragg reflections at d spacings of 5.10 A, 3.61 A, and 2.56 A, 
respectively, from a face-centered square cell with an edge of a = 10.2 A. The broad 
background centered at ca. Qxy = 1.41 A" 1 and the shoulder at 1.58 A _1 are likely due to 
the poorly organized alkyl chains. 7879 The isolated (4,0) peak was fit to a Lorentzian 
function and yielded a full width at half maximum (Q^ ) of 0. 1 A" 1 Insertion of this 

•'fivhm 

value into the Scherrer equation, 147 £ = [(1 .87t) / (Q^ )], yields an average crystalline 



80 



1.0 - 



o 

N 



s 

O 

Z 



0.5 - 



0.0 



" (2,0) 

+ H 

* 4 

+ 

+ 

+ + + 


(4,0) 

+ + 

++ 
+ 
+ 

: 


+ 

t 


+ + 

V ) l 



1.0 1.5 2.0 

Q„ (A" 1 ) 



2.5 



3.0 



Figure 4-6. The difference between the field cooled and zero field-cooled magnetization, 
DM, is shown as a function of temperature. Typical data from the Mn-rich (i.e. x > 0.25) 
samples are shown when the magnetic field, for measuring and field cooling, was 100 G. 



coherence length (£) of -60 A, or 5 unit cell lengths, indicating that the 2D networks 
cover an average area of approximately 3600 A 2 . 
Magnetic Properties 

The magnetic properties of a 10 cm 2 sample containing 300 bilayers (150 bilayers 
per side) of the nickel-iron-cyanide network transferred to Mylar were investigated by 
SQUID magnetometry. Two measurements were performed, one with the sample 
surface oriented parallel to the magnetic field and one with the sample surface oriented 
perpendicular to the magnetic field. The background corrected field-cooled 
magnetization versus temperature obtained in a field of 20 G is shown in Figure 4-7. The 
rise in magnetization below T c = 8 K observed in both orientations is attributed to the 
onset of ferromagnetic order. The magnetic behavior is clearly anisotropic, with the 
sample displaying a stronger magnetic response when the surface is oriented parallel to 



81 




10 15 20 25 
T(K) 



30 



Figure 4-7. The temperature dependence of the magnetization after field cooling in 20 G 
with the sample surface aligned parallel (filled circles) and perpendicular (open circles), 
to the magnetic field. The measuring field was 20 G. The break at Tc = 8 K is indicative 
of long-range ferromagnetic order between the Fe3+ (S = V2) and Ni2+ (S = 1) centers. 



the magnetic field. The presence of a ferromagnetic state at low temperature is further 
supported by the magnetization vs. field data taken at 2 K. The sample shows a rapid 
increase in magnetization at low field followed by a gradual approach toward saturation 
at higher fields. Cycling the magnetic field at 2 K results in the hysteresis loops 
(corrected for the diamagnetic background) shown in Figure 4-8. The plots are 
normalized to the magnetization at 5 T. Again, there is clear anisotropy in the magnetic 
behavior between the two orientations of the sample with respect to the field. When the 
field is parallel to the sample surface, the magnetization increases more rapidly with 
respect to the field and the remnant magnetization is 35% versus 8% in the perpendicular 
orientation. The coercive field is also slightly anisotropic, being 140 G in the parallel 
orientation and 1 10 G in the perpendicular orientation. 









82 






1.0 



0.5 - 



£ 0.0 



-0.5 - 



•1.0 



■ 






-1.0 -0.5 0.0 0.5 

H (kG) 



1.0 



Figure 4-8. Hysteresis loops measured at 2 K with the sample surface aligned parallel to 
(filled circles) and perpendicular to (open circles) the applied magnetic field. The 
magnetization is normalized to the saturation magnetization. 



Discussion 
Choice of System and Monolayer Behavior 

Designing a system to result in the formation of a coordinate-covalent network at 
the air-water interface requires the appropriate transition metal complex building blocks. 
Octahedral transition metal ions possessing linear bridging ligands are well suited to the 
assembly of square-grid networks since the required 90° bond angles are inherently 
present. By substituting one position with a hydrophobic ligand, the building block can 
be made amphophilic and thus tailored for assembly reactions at the air-water interface. 
Condensation of the amphiphilic building block can then be accomplished by reaction 
with a suitable aqueous transition metal ion contained in the subphase. Confinement of 
the reacting system to the water surface directs the resulting structure to a two- 
dimensional motif. 



83 

The numerous examples of pentacyanoferrate complexes and the substitutional 
inertness of the cyanide ligand make this class of compounds well suited to our assembly 
strategy. Multilayer films of the single chain derivative of 3, (4-octadecylamino- 
pyridine)pentacyanoferrate(III) (4) has previously been described. 189 The sodium salt of 
4 was reported to be too soluble for film preparations, but when prepared as a mixed film 
with hexadecyltrimethylammonium counterions, stable Langmuir monolayers resulted. 
No difficulties with hydration of the single-chain complex were reported. 

We decided to modify the pentacyanoferrate complex to the dual chain derivative 
to match more closely the size of the amphiphilic ligand with that of the metal complex 
head group. The addition of a second alkyl chain also decreased the solubility of the 
complex and eliminated the need for long chain alkyl ammonium counterions. It was 
found to be beneficial to exchange the sodium counterions for tetramethyl-ammonium 
ions to decrease the hygroscopic nature of the complex and to aid its dissolution in 
chloroform. The resulting complex forms a Langmuir monolayer that creeps slowly on 
water (Figure 4-2), but forms a highly stable film after reaction with aqueous Ni 2+ ions to 
form an insoluble polymeric network. 

Evidence for the condensation reaction is seen in-situ at the air/water interface. In 
the absence of Ni + , 3 forms a liquid expanded phase upon compression. This behavior is 
reasonable, as amphiphiles with twelve-carbon alkyl tails do not normally form 
condensed phases at the air-water interface at room temperature. 190 Upon addition of 
Ni , a condensed phase is seen in the pressure vs. area isotherm and in Brewster angle 
microscopy. The mean molecular area of 52 A 2 at the onset of pressure correlates with 



84 

the limiting area per molecule of the complex over pure water and suggests that the film 
is highly condensed at zero pressure over the Ni 2+ subphase. 

In a mixed-metal cyanide square grid network (Scheme 4-1), a centered unit cell 
will have two iron amphiphiles per unit cell. Doubling the area per molecule, determined 
from the pressure vs. area isotherm, gives a cell area of 104 A 2 , which then corresponds 
to a cell edge distance of 10.2 A. This value is in agreement with the 10.2 A 2 determined 
for the cell edge by GIXD and suggests that the mean molecular area is determined by the 
lattice spacing of the inorganic two-dimensional grid network and not by the Van der 
Waals interactions of the organic chains. 

Furthermore, BAM and surface pressure data indicate that the nickel-iron-cyanide 
network forms with or without preorganization of the monolayer. That is, the MMA 
obtained by compression of the iron amphiphile over a Ni 2+ subphase is very close to the 
MMA obtained after injection of a Ni 2+ solution under an organized monolayer of the 
iron complex. A condensed film is formed at zero pressure over the Ni 2+ subphase, and 
subsequent compression of the film only acts to push together domains that have already 
assembled at the interface. 
Structure of the Network 

The formation of an extended two dimensional array is dependent on the 
exclusive bridging of the four in-plane cyanide ligands, as bridging of the trans cyanide 
ligand would effectively terminate the structure and result in an amorphous inorganic 
polymer. Evidence for a well-organized network from GIXD, XAFS, and FT-IR suggests 
that while coordination of the axial cyanide is possible, this mode is most likely labile in 
the absence of the added stability brought on by extended bridging interactions. 



85 

The results of the GIXD clearly show the presence of a structurally coherent 
inorganic network in the mixed metal film. The three peaks shown in Figure 4-6 fit very 
well to the expected (h k) pattern for a face-centered square grid network. The unit cell 
edge length of 10.2 A deduced from the diffraction data is very similar to that reported in 
cubic Prussian blue derivatives. 191 The high background scattering near 1.4 A" 1 and 1.6 
A" 1 is in the range of Q x -y normally seen for alkyl chain packing and suggests that the 
alkyl chains are loosely organized. This observation would be expected in light of the 
large area per alkyl chain in the condensed network. 

The XAFS data complement the conclusions of the GIXD experiments. The 
Ni-N, C-N, and Ni-C distances of 2.09 A, 1.13 A, and 3.22 A, respectively, were 
obtained from modeling the Ni edge XAFS. Combining these bond lengths with the 
average Fe-C bond length of 1.95 A reported for other Fe-CN-Ni bridged systems, 3840 
leads to a Ni-Fe separation of 5. 17 A. This value is close to the 5. 10 A separation 
deduced from the GIXD. The quality of the XAFS fit supports the modeled nickel 
coordination environment in which the octahedral nickel ions are coordinated in-plane by 
the nitrogen terminus of the cyanide bridge and the axial sites by oxygen, most likely 
from coordinated water. 
Magnetism 

The formation of a structurally coherent inorganic network at the air- water 
interface is confirmed by the transition to a ferromagnetic state below 8 K in the 
multilayer film containing 150 bilayers per side. The ability of the cyanide ligand to 
mediate magnetic exchange between two paramagnetic metal ions is well known and has 
been extensively explored in cubic Prussian blue derivatives. In particular, the Fe 3 7Ni 2+ 
Prussian blue 192 ' 193 was found to be ferromagnetic with a T c of 23 K. In addition, 



86 

ferromagnetic exchange has also been reported in a series of two-dimensional cyanide- 
bridged iron-nickel compounds with T c 's on the order of 10 k. 38194 The ferromagnetic 
behavior of these materials is rationalized 195 by realizing that for octahedral metal 
centers, the magnetic orbitals are the Fe 3+ (S = V2) t 2g and the Ni 2+ (S = 1) e g sets, and that 
the cyanide orbitals that overlap with each of them are orthogonal. 

For the Fe 3 7Ni 2+ LB film system, the ordering temperature of 8 K is lower than 
the T c of 23 K observed in the cubic analogue, and is more similar in magnitude to the 
ordering temperature reported in other low dimensional Fe-CN-Ni networks. 196 Lower 
ordering temperatures for the 2D systems relative to the cubic analogues is expected as 
the number of exchange pathways per magnetic ion is reduced. Further evidence for a 
two dimensional network is obtained from the anisotropic magnetic behavior seen in the 
film. The stronger magnetic response of the sample when oriented with the surface 
parallel to the magnetic field suggests a magnetic easy axis within the plane of the 
network. A strict analysis of a magnetic vector in the film is limited though due to 
uncertainties in how the microscopic surface roughness of the substrate affects variations 
in the orientation of network sheets relative to the plane defined by the macroscopic 
substrate. The high anisotropy of the magnetization does discriminate against the 
magnetic behavior arising from cubic Prussian blue-like particles and is highly suggestive 
of a low dimensional system. 197 More detailed studies on the magnetic properties of the 
LB film networks are ensuing since the unique structural features of these monolayer 
networks may provide experimental probes of the exchange coupling interactions in 
metal cyanide networks and the how the issue of dimensionality influences ordering in 
mixed-spin 2D systems. 198199 



87 

Mechanism and Structure Directing Elements 

The two-dimensional nickel-iron-cyanide network forms at the air/water interface, 
but does not require pre-organization of the amphiphiles. The condensation reaction 
proceeds in the absence of applied surface pressure when the amphiphile 3 is spread on 
the Ni + containing subphase, in which case subsequent reduction of the surface area 
simply compresses the preformed domains. Compression of the film thus appears to do 
little to extend the in-plane order of the networks, and instead, only works to increase the 
density of the domains allowing for better transfer of the networks to solid supports. Pre- 
organization of the amphiphile, followed by injection of Ni 2+ ions into the subphase 
results in the same network, with no significant difference in domain organization. 

Control of the reaction to form the square grid network results from the 
orientational constraints of the octahedral metal complex with linear cyanide bridging in 
combination with the interface as a structure-directing element. This view is supported 
by attempts to form the same networks from solution. The analogous reaction of 3 with 

•2+ ■ 

Ni in methanol yields an insoluble precipitate, which is shown by X-ray diffraction to 
be amorphous. When compared to the homogeneous reaction, the air-water interface not 
only directs the structure of the final material, but also acts to enhance the structural 
coherence length as well. 

It is interesting to compare the mechanism of formation of the metal cyanide two- 
dimensional networks to other examples of Langmuir-Blodgett films that contain 
inorganic networks. For example, there are now several examples of metal phosphonate 
based LB films, where the inorganic extended solid networks determine the in-plane 
structures. 109117145 The difference is that for the metal phosphonates, the LB films form 
with the same structure that forms in the solid-state. The structure is determined by the 



88 

inorganic lattice energy. With the metal phosphonates, the LB film processing directs 
where the structure will form and affords control of the fabrication to one layer at a time, 
but the air/water interface does not act as a structure-directing element. In contrast, the 
iron-cyanide-nickel network described here does not form from compound 3 in the 
absence of the interface. The interface directs where the reaction will take place and 
limits the reaction to one layer at a time, but importantly, it also directs the structure. 

Conclusions 
An amphophilic octahedral iron complex containing linear cyanide ligands was 
designed and synthesized as a building block for the assembly of two dimensional square 
grid nickel-iron-cyanide networks at the air- water interface. The reaction of Langmuir 
monolayers of this complex with aqueous nickel ions contained in the subphase results in 
the formation of coordinate covalent networks. Characterization of these networks by 
various techniques indicates that the structure is two-dimensional and coherent over an 
average domain size of 3600 A 2 . Magnetic measurements indicate a ferromagnetically 
ordered state below 8 K with the magnetic behavior highly dependent on the orientation 
of the sample with respect to the field. This synthetic method demonstrates that the air- 
water interface can function as a structure-directing element in the assembly of new 
supermolecular network solids and, in addition, provides a means for transferring these 
materials in a controlled fashion to solid supports. This assembly strategy may aid in the 
future development of nanoscale materials and with interfacing them at a surface. 



CHAPTER 5 
INTERFACIAL ASSEMBLY OF CYANIDE-BRIDGED FE-CO AND FE-MN 

SQUARE GRID NETWORKS 






Introduction 

Coordination chemistry routes to finite and infinite networks make use of the 
predictable directional characteristics of coordinate covalent bonds. ' " 
Tunable variables like stochiometry, template additives, secondary structure building 
blocks, or kinetic control are used to determine the final network structure, and several 
examples are included in the current issue. Potential applications of inorganic finite and 
infinite networks include recognition and sensing, catalysis, electronic and optical 
functions, and magnetic effects related to information storage. It is interesting to 

note that several of these applications are likely to involve positioning at surfaces, and 
routes to locate the finite or infinite networks at interfaces will be needed." 
One approach is to involve the interface directly in the assembly, to carry out the network 
fabrication where it will be located. In this case, the interface can play a role in 
determining the network structure. Examples of assembly at liquid interfaces have been 
published, including some two-dimensional infinite networks. ' ' ' ' ' ' The 
surface of a liquid retains the structure directing character of an interface, but at the same 
time is fluid and can facilitate diffusion of reactants. Careful understanding of these 
processes is now possible largely as a result of surface sensitive characterization 
methods, including grazing incidence X-ray diffraction as detailed in a recent review. 



89 



90 



CH 3 CB, 
(CHjhi (CHj)„ 



r. 9 ■ 


2- 




N*. N -jN 




OH 2 


N^JJX 


+ 


H.-0* v | >H ; 

H,0*^ | ^OH 2 
OHj 


L N 




- 



1 

(Langmuir monolayer) 



M = Co 2 %Mn 2+ 
(subphase) 



"A'A" 



"WW? ^P^ \^ A y 



square-grid network 
(air-water interface) 



Scheme 5-1. Assembly of two-dimensional grid networks at the air- water interface. 



We recently reported the fabrication of an Fe 3 7Ni 2+ mixed-metal cyanide- 
bridged square grid network at the air-water interface, showing that the interface can act 
as a structure directing entity when preparing coordinate covalent networks. 205 In this 
paper, we show the process is general and describe two new examples of square grid 
networks prepared as monolayers at the air/water interface. The technique, outlined in 
Scheme 5-1, uses the air/water interface and involves the reaction of an amphiphilic 
pentacyanoferrate (3+) complex (1) confined to a monolayer on a aqueous subphase 
containing a second divalent metal ion. By confining one of the reactants to the air/water 
interface the propagation of the structure in the third dimension is prevented, resulting in 
a planar network at the water surface. The effect of the interface works in tandem with 
the defined bond angles of the octahedral metal complexes and the linear geometry of the 
cyanide bridge to direct the final structure of the network to a face-centered square grid 
array. The same reactants in a homogeneous reaction give amorphous colloidal products, 
thus illustrating the ability of an interface to direct the structure of the network. Also, the 
interface-assembled network can be conveniently transferred to solid supports by the 






91 

Langmuir-Blodgett technique, permitting added structural and materials property 
characterization. 

The cyanide ligand is particularly attractive for use in network assembly. Its 
linear geometry and ambidentate nature make for a versatile building block when 
combined with various transition metal complex geometries. 13 ' 19 ' 21 ' 22 ' 38 ' 51 * 52 ' 169 " 172 In 
addition, cyanide has been shown to mediate both magnetic and electronic exchange 
between the bridged metal centers, giving rise to materials with interesting physical 
properties, including a family of molecule-based magnets. 51 " 53 ' 169 ' 173 ' 174 We previously 
reported an Fe 3 7Ni 2+ cyanide bridged network prepared at the air/water interface and 
showed it to be magnetic. 205 This study extends the series to include Fe 3+ /Co 2+ and 
Fe + /Mn + mixed metal cyanide networks. These new two-dimensional networks are also 
magnetic, and their behavior is compared to related three-dimensional hexacyanometalate 
complexes. 

Experimental Section 

Materials. The amphiphilic complex tetramethylammonium pentacyano(4- 
didodecylaminopyridine)ferrate(III) 6H 2 was prepared as previously described. 205 
Attenuated total reflectance (ATR) FT-IR samples were prepared as monolayers on clean 
silicon ATR crystals. Grazing incidence X-ray diffraction (GIXD) samples were 
prepared on petrographic slides that were first cleaned using the RCA procedure 178 and 
made hydrophobic by deposition of a monolayer of octadecyltrichlorosilane. 179 ' 180 
Samples for SQUID magnetometry measurements were prepared on Mylar (Dupont) 
substrates cleaned with absolute ethanol prior to use. 

Film preparation. The amphiphilic iron complex 1 was spread onto the water 
surface from a chloroform solution. Multilayer films of the iron-cyanide-manganese and 






92 

iron-cyanide-cobalt networks were transferred as Y-type films onto hydrophobic 
substrates at a surface pressure of 25 mN/m over a subphase 1 g/L in MnCNChVxHjO or 
Co(NC>3)2 xtbO at ambient temperature. Transfer ratios for both the upstrokes and 
downstrokes were between 0.85 and 1 for all layers. 

Instrumentation. IR spectra were collected using a Mattson Instruments 
(Madison, WI) Research Series- 1 FTIR spectrometer with a deuterated triglycine sulfate 
(DTGS) detector. The LB films were prepared by using a KSV Instruments 5000 trough 
modified to operate with double barriers. The surface pressure was measured with a filter 
paper Wilhelmy plate suspended from a KSV microbalance. Subphase solutions were 
prepared from 17.8-18. 1 MQ cm water delivered with a Barnstead Epure system. 
Magnetization measurements were performed on a Quantum Design MPMS SQUID 
magnetometer. GIXD experiments using synchrotron radiation were performed at the 
Advanced Photon Source, Argonne, IL, at the Materials Research Collaborative Access 
Team beamline (sector 10). 76 ' 205 The GIXD scans were performed on LB films 
transferred to glass slides. The sample was positioned in the center of an 8-circle Huber 
goniometer and oriented at an angle of 0. 13 degrees relative to the incident beam. The 
incident beam was collimated to 200 microns high by 1500 microns wide and tuned to a 
wavelength of 1 .254 A. Diffracted intensity in the xy plane was measured using a Nal 
scintillation counter mounted on the Huber goniometer. The diffracted signal was 
collimated prior to the detector using Soller slits giving an experimental resolution on the 
order of 0.0 15 A" 1 . 



93 




(a) 



(b) 

Figure 5-1. BAM images taken at zero surface pressure of complex 1 over (a) pure 
water and over (b) a 1 mmol Mn2+ subphase. 



Results and Discussion 
Langmuir Monolayers 

Evidence for the condensation reaction is first seen directly at the air/water 
interface using the traditional Langmuir monolayer methods of Brewster angle 
microscopy (BAM) and pressure vs. area isotherms. At room temperature, the 
amphiphilic pentacyanoferrate (3+) complex 1 is in a liquid expanded phase on water 
(with NaCl), forming two-dimensional bubbles at the interface in the absence of applied 
pressure (Figure 5-la). As the film is compressed, a continuous film forms. The pressure 
vs. area isotherm for 1 does not show evidence for a phase transition with increased 
pressure, indicating that 1 maintains the liquid expanded phase up until collapse. 

If a complexing metal ion (Mn 2+ or Co 2+ ) is added to the subphase, the amphiphile 
behaves very differently, forming a condensed phase at all pressures. The monolayer 
must be compressed to a much smaller area before the surface pressure increases 
(Figure 5-2). After the surface pressure begins to rise, the slope is much sharper than in 
the absence of complexing ions, reflecting the lower compressibility of the film. The 



20 



94 



50 








«•■». 


<* 




. 


J 40 


■ %v A 




■ 


Z 


^ A 






s 


b A 




1 


^ 30 


O A 




■ 


E 

3 


° A 




, 


■ 


O A 






I 20 


A 




■ 




O A 
O A 




, 


« 


Q. A 






i 10 


id A 




• 


•S 


% \ 






E 


Si ^v 




1 


80 


^s V 










, 



40 



60 



80 



100 120 



Mean Molecular Area (A ) 



Figure 5-2. Room temperature surface pressure vs. mean molecular area isotherms 
forcomplex 1 over pure water (triangles) and lg/L Co(NC>3)2 (circles). 



condensed phase over Mn 2+ is seen in the BAM image, Figure 5- lb. The behavior is 
consistent with cross linking of the amphiphiles by the subphase metal ions through 
cyanide bridges to form a network. The limiting area for the condensed films at low 
applied pressures is between 50 and 60 A 2 /amphiphile, which will be shown, below, to 
agree with XRD data. 
Infrared Spectroscopy 

The network monolayers can be transferred onto solid supports using traditional 
LB deposition procedures, permitting further structural and physical property 
characterization. Evidence for cyanide bridging is seen by FTIR. Attenuated total 
reflectance FTIR spectra of the cyanide stretches for the condensed films are compared in 
Figure 5-3 to those of (1), obtained as a KBr pellet. The spectrum of complex (1) shows 
a band at 21 1 1 cm" 1 with a shoulder at 2128 cm" 1 and is in the typical range with the 



95 



5 
< 

o 

e 

•- 
o 

< 





I 




'. 




' 


-^■""■"••h. S 


^< 


(a) 


jr\^ 




(b) ; 




' 


/ 


V 


(c) 




-i 1 — i — ■ — i — i — u 





2200 



2150 2100 2050 2000 
Wavenumber (cm ) 



Figure 5-3. Infrared absorbance spectra of the C-N stretching region for monolayer films 
on Si ATR crystals of the grid network formed from the reaction of complex 1 with (a) 
Co2+ and (b) Mn2+ compared to (c) the spectrum of 1 as a KBr pellet. 



expected splitting for an Fe 3+ pentacyanide complex. The Fe-Mn film shows a broad 
band centered at 2145 cm" 1 and agrees well with the CN stretching frequency reported for 
the related Mn 3 [Fe(CN) 6 ] Prussian blue analogue, 206 confirming the presence of Fe-CN- 
Mn bridging in the film. The FT-IR spectrum for the Fe-Co film is more complex and 
shows a split band with a peak at 2155 cm" 1 and a broad peak at 2090 cm" . A similar 
splitting has been reported in a Co[Fe(CN) 6 ] Prussian blue analogue and has been 
attributed to the presence of two different oxidation states of the iron cyanide complex. 
The peak at higher wavenumbers is due to an Fe 3+ -CN-Co 2+ bridge and the lower energy 
peak to an Fe 2 "-CN-Co 2+ bridge. The band reported at 2133 cm' 1 for Co[Fe(CN) 6 ] 
attributed to an Fe 2+ -CN-Co 3+ bridge is not observed in the monolayer Fe-Co film. The 
IR data suggest that some Fe 3 * is reduced in the network, as in the three-dimensional 



96 

analog, but the relatively large excess of Co 2+ in the subphase assures that any Co 3 * that 
forms in the network is quickly reduced, leaving only Co 2+ in the film. 
Grazing Incidence X-ray Diffraction 

The in-plane structure of the networks is confirmed by X-ray diffraction. The 
small quantity of material and strong background scattering make conventional X-ray 
sources ineffective for characterizing the in-plane structure of thin films. However, the 
combination of enhanced X-ray flux from synchrotron radiation with grazing incidence 
angles reduces the signal to noise ratio to a level where scattering from as little as a 
monolayer film can be detected. This method of grazing incidence X-ray diffraction 
(GIXD) has been described in detail. 78 The GIXD patterns obtained on 15-bilayer 
samples of the Fe-Mn and Fe-Co networks transferred to glass slides are shown in 
Figure 5-4. Both patterns show the same three peaks, with slight shifts in spacing, and 
confirm that the Fe-Mn and Fe-Co networks are isostructural. The diffraction peaks for 
the Fe-Mn film correspond to lattice spacings of 5.18 A, 3.69 A, and 2.61 A. The 
analogous spacings for the Fe-Co film are 5.10 A, 3.62 A, and 2.55 A, respectively. Both 
patterns can be indexed to a face centered square network with Miller indices of (20), 
(22), and (40), in order from large to small spacings, and yield face-centered square unit 
cells of a = 10.36 A for the Fe-Mn and a = 10.20 A for the Fe-Co networks. Analysis of 
the peak widths of the (20) and (40) reflections in both films by application of the 
Scherrer equation yields a structural coherence length of approximately 80 A for both 
films. 



97 



< 



o 
U 




6 5 4 3 

d spacing (A) 

Figure 5-4. Grazing incidence X-ray diffraction patterns for 10 bilayer samples on glass 
of the (a) Fe-Mn and (b) Fe-Co grid networks. Both patterns can be indexed to a face- 
centered square grid network with cell edges of 10.4 A (a) and 10.2 A (b). 



Magnetism 

The three-dimensional analogs Mn3[Fe(CN) 6 ]2 and Co3[Fe(CN)6]2 are low- 
temperature ferrimagnets with T c of 9 K and 14 K, respectively. 192 Magnetic exchange in 
these compounds is mediated by the cyanide ligand. The temperature dependent 
magnetization for the two-dimensional assemblies is reported in Figure 5-5 for the Fe-Co 
film and in Figure 6 for the Fe-Mn film, each in two orientations. The Fe-Co sample was 
100 bilayers per side and the Fe-Mn sample 125 bilayers per side. Both films were on 10 
cm of Mylar substrate. The magnetic behavior of each film is consistent with the 
ferrimagnetic exchange observed for the three-dimensional parent compounds, although a 
significant diamagnetic background contribution from the Mylar substrate makes it 
difficult to unambiguously quantify the moments to discern ferrimagnetism from 









98 



3 

J 

O 






4.0 


T ' ' ■ ■ 1 ' ■ ■ ■ 1 ■ ■ ■ ■ 1 

A 
A 




A 




. A A 


2.0 


A 


0.0 


A 
A 

'^■saaa© ooooo o AAAAA cA A £ 







10 



20 
T(K) 



30 



40 



Figure 5-5. The temperature dependence of the product of the zero-field magnetization 
(measured in 20 G) and temperature for a 100 bilayer (per side) sample of the Fe-Co 
network on Mylar showing the anisotropy of the magnetic response when the field is 
applied parallel to the sample surface (triangles) and perpendicular to the sample surface 
(circles). 



ferromagnetic exchange in the transferred films. The magnetic response of each film is 
anisotropic with respect to the sample orientation in the applied magnetic field. The Fe- 
Co film shows negligible magnetic response (Figure 5-5) when the field is applied 
perpendicular to the magnetic planes. Conversely, magnetization increases rapidly below 
10 K with the applied field parallel to the plane of the network. The magnetic response 
of the Fe-Mn film (Figure 5-6) is also anisotropic, but in the opposite sense. The 
variation in the orientation of the magnetic easy axes must reflect the reduced symmetries 
of the crystal fields in the two-dimensional networks and the differences in single ion 



99 



4.0 



I 

2 20 ■ 

H 

« 

5 



0.0 



■ I 



I I I I 



o 

o 

o 

o 

o 

o 

o 

o 




A °°° 

A A A A A 



o 
A 



a £ 2 







10 



20 
T(K) 



30 



40 



Figure 5-6. The temperature dependence of the product of the zero-field cooled 
magnetization (measured in 20 G) and temperature for a 125 bilayer (per side) sample of 
the Fe-Mn network on Mylar showing the anisotropy of the magnetic response when the 
field is applied perpendicular to the sample surface (circles) and parallel to the sample 
surface (triangles). 



anisotropics expected for Co 2+ , Mn 2+ , and low spin Fe 3+ . The pentacyanoferrate(3+) 
complex common to both structures is a low-spin d 5 ion, known to experience significant 
spin-orbit coupling. 208 In addition, the 4-aminopyridine ligand and bridging of the in- 
plane cyanides lowers the symmetry of the Fe 3+ site. When coupled with the isotropic S 
= 5/2 Mn + ion, the ferric site can be expected to define the magnetic easy axis. In the 
case of the Fe-Co material, the strong Ising character of the Co 2 * ion dominates, in this 
case confining the moments to the network plane. The anisotropy of the Fe-Co film is 
similar to that observed for the analogous Fe-Ni film that was described previously. 205 

When the monolayers are transferred to solid supports for magnetic studies they 
form bilayers with the metal cyanide networks depositing face-to-face. Each inorganic 
bilayer is then separated from the next by the alkyl tails of the amphiphilic aminopyridine 



100 

ligand. The exact nature of the interaction between the face-to-face networks is not yet 
clear, nor is its influence on the anisotropic response. More detailed magnetic studies 
comparing true monolayers with the multilayer films are underway. Nevertheless, 
magnetic exchange in the films provides additional evidence that extended networks form 
in the condensation reaction at the air/water interface. In addition, the anisotropic 
magnetic behavior discriminates against a cubic Prussian blue-like product and is 
consistent with a planar structure. 

Conclusions 

Reaction of an amphiphilic pentacyanoferrate(3 + ) complex at the air/water 
interface with divalent metal ions from the subphase results in two-dimensional cyanide- 
bridged coordinate-covalent networks. Confining one of the reactants to the surface of 
water illustrates the concept that the interface can be used as a structure-directing element 
for preparing two-dimensional arrays. The same reaction in the absence of the interface 
generates amorphous colloids. The cyanide bridges mediate magnetic exchange, just as 
in the related three-dimensional hexacyanometalate analogs, but the two-dimensional 
networks lead to anisotropic behavior that changes with the identity of the metal ions. 






CHAPTER 6 
FERROMAGNETISM AND SPIN-GLASS BEHAVIOR IN LANGMUIR-BLODGETT 
FILMS CONTAINING A TWO-DIMENSIONAL IRON-NICKEL CYANIDE SQUARE 

GRID NETWORK 



Introduction 

Metal cyanides are a structurally diverse class of materials. The best known 
examples come from the family of "Prussian blues" (named after the mixed-valent deep 
blue Fe in [Fe n (CN) 6 ]) which consist of octahedral metal ions bridged through [M(CN) 6 ] n " 
to form three-dimensional cubic solids. 209 However, many other structural motifs have 
been realized by introducing "blocking ligands" to one or both of the metal complex 
building blocks, thereby lowering their symmetry and the symmetry of the final cyanide- 
bridged arrays. This strategy has yielded an array of structures over a wide range of 
dimensionality including clusters, 18 " 25 molecular squares, 26 ' 27 linear chains, 30 " 32 
ladders, "' as well as quasi-two-dimensional square-grid 37 " 40 and "honeycomb" 
networks. 41 " 43 - 210 ' 211 

Aside from their structural diversity, metal-cyanides also show interesting 
magnetic behavior. For many examples, the nature of the magnetic exchange can be 
anticipated in advance from basic orbital interaction arguments and the predictable 
structure-directing quality of the cyanide bridge. This inherent ability to tailor both the 
structure and magnetic exchange in metal-cyanide systems makes this family of materials 
well suited for studying molecule-based magnetic phenomena. Indeed, a wide range of 



101 



102 




1 



LB 




.,:•< 



N N 



R ? R H 

M 1/ 



T-r 



I N^-M | j F« \~rpfH- 

-7=»Fe— ^.Nl^- I 



I 



I 



I Z^Hf^ ^==Fe= -^Nl^ 

I -?»F* j t S r"^ \ ^ f^= | 



I 



-M. 



K R R R 



FeNi-bi 



FeNi-mono 



Scheme 6-1. Reaction of a 1 as a Langmuir monolayer with aqueous Ni2+ ions in the 
subphase results in a two-dimensional Fe-CN-Ni grid network. Controlled transfer of the 
reacted film to solid supports results in an isolated monolayer (FeNi-mono) when the 
support is drawn out of the subphase (hydrophilic transfer) or a single bilayer (FeNi-bi) 
when the hydrophobic support is dipped down through the water surface and withdrawn. 
The FeNi-150 sample is prepared by repeating the bilayer procedure through 150 cycles. 



magnetic phenomena have been observed in metal cyanides including high-spin 
clusters, 18 " 25 metamagnetism, 39,43 ' 196 ' 211 " 214 room temperature magnetic 
ordering, 51 ' 173174 - 215 and spin-glass behavior. 216 ' 217 

We have recently described the synthesis of a cyanide-bridged Fe m -Ni n square 
grid network at the air- water interface and the transfer of these networks to solid supports 
by the Langmuir-Blodgett (LB) technique. 205 The 2D network can be transferred as an 
isolated monolayer, as a single bilayer, or as multiple bilayer assemblies (Scheme 6-1). 



103 

Initial investigations into the magnetic properties of the multiple bilayer films comprised 
of these Fe-Ni networks by DC magnetometry revealed a ferromagnetic state below 8 K. 
Single-layer control over the deposition process provides an opportunity to observe how 
the magnetic properties of the system evolve as it changes from a monolayer to a bilayer 
to a multilayer film. We report herein a detailed investigation of the magnetic properties 
of this 2D system using both DC and AC magnetometry which reveals both 
ferromagnetic and spin-glass behavior at low temperatures. 

Experimental 
Materials. Nickel nitrate hexahydrate (99%) was purchased from Aldrich 
(Milwaukee, WI) and used as received. The amphiphilic complex (1) Bis(tetramethyl- 
ammonium) pentacyano(4-didodecylaminopyridine)ferrate(III) 6H2O was prepared as 

90S 

described. Subphase solutions were prepared from 1 7.8-1 8. 1 MQ cm water delivered 
with a Barnstead Epure system. 

Film preparation All films were prepared on Mylar substrates precleaned with 
absolute ethanol. The 150 bilayer-per-side sample was prepared as previously 
described. Briefly, a Langmuir monolayer of complex 1 is reacted at the air water 
interface over a subphase containing 1 g/L of Ni(N0 3 ) 2 6H 2 (see Scheme 6-1). The 
resulting square grid network that forms is transferred at a surface pressure of 25 mN/m 
as a y-type film by the Langmuir-Blodgett technique. The transfer ratio (T R ) of the 150 
bilayer sample (FeNi-150) was T R = 1.0 ± 0. 1 for both the upstrokes and downstrokes 
throughout the transfer process (where T R = 1.0 signifies a complete monolayer transfer). 
A separate sample consisting of a single bilayer (FeNi-bi), i.e. one downstroke followed 
by one upstroke, was prepared on a Mylar surface pre-coated with 5 bilayers of 



104 

octadecanol (for increased hydrophobicity). The transfer ratio for FeNi-bi was T R = 1.0 
± 0.1 for both the downstroke and upstroke. A sample consisting of a single monolayer 
of the Fe-Ni grid network (FeNi-mono) was prepared using a single transfer starting with 
the Mylar immersed in the subphase, i.e. hydrophilic transfer. The transfer ratio for 
FeNi-mono was T R = 1 .0 ± 0. 1 . 

For magnetic measurements, each sample measuring 10 cm 2 was cut and packed 
into gel caps for SQUID magnetometry. The rectangular pieces were packed parallel to 
one another and oriented with the plane of the sample surface aligned parallel (//) or 
perpendicular (1) to the applied DC or AC magnetic fields. Background corrections 
were applied by subtraction of the diamagnetic signal measured on a similar (within 3%) 
mass of clean Mylar and sample container. 

Instrumentation. The LB films were prepared by using a KSV Instruments 5000 
trough modified to operate with double barriers. The surface pressure was measured with 
a filter paper Wilhelmy plate suspended from a KSV microbalance. Magnetization 
measurements were performed on a Quantum Design MPMS SQUID magnetometer. For 
the AC susceptibility, Xac, all measurements of the in-phase (real or dispersive) 
susceptibility, %\ and out-of-phase (imaginary or absorptive) susceptibility, %", were 
made after zero field cooling the sample, with subsequent warming under zero applied 
DC field and an oscillating AC field of 4 G. 



105 

Results 
Film Structure 

The two-dimensional grid network prepared as shown in Scheme 6-1 has been 
structurally characterized by grazing incidence x-ray diffraction, x-ray absorption fine 
structure, infrared spectroscopy, and x-ray photoelectron spectroscopy. 205 The in-plane 
structure of the network consists of a face-centered square grid array of low spin Fe m 
ions that are bridged through cyanide to Ni 11 ions. The in-plane lattice parameter is a = 
10.4 A, which yields an Fe-Ni separation of 5.2 A. The average structure coherence 
length, as determined from x-ray diffraction, is approximately 6 unit cell lengths. This 
gives an average coherent particle size on the order of 3600 A 2 , which would contain 
approximately 144 ions (72 Fe 111 ions and 72 Ni 11 ions). 

As depicted in Scheme 6-1, the networks can be transferred to solid supports in a 
controlled fashion by the LB technique. If the dipping cycle begins with the substrate 
submerged, withdrawal of the substrate will result in the transfer of a single monolayer of 
the Fe-CN-Ni network (FeNi-mono) oriented such that the inorganic network is in direct 
contact with the substrate surface. In this case, the planar network is an isolated two- 
dimensional system (it is not truly isolated, as a large anisotropic background arises from 
the substrate). On the other hand, if the substrate begins above the water surface, one 
dipping cycle of passing the substrate into the subphase and back out results in a bilayer 
(FeNi-bi), with the organic portion of the material in contact with the substrate and 
inorganic networks face-to-face, sandwiched in the center of the bilayer. The nature of 
the bonding interaction in the interfacial region between the networks is uncertain, but 
likely contains a mixture of covalent bonding via coordination of the axial cyanide of the 
iron complex to Ni 2+ ions in the adjacent layer, hydrogen bonding via intercalated water 



106 



15 



i 



10 



5 - 



- 



-i — i - 1 — i | i i — i i | — i i i i 

H and H = 100 G 

FC nieas 






T*pvm A A 



Q. 



. , v w o o j 
-i — i — i — i — l — i — i i i I i i i i 



* 6 A < 



10 



15 



T(K) 



Figure 6-1. Background corrected M(7) for FeNi-150 aligned parallel to the applied 
field [(o) field-cooled and (•) zero field-cooled] and perpendicular to the applied field 
[(a) field-cooled and (A) zero-field cooled]. Cooling fields and measuring fields were 
each 100 G. 



molecules, or simple electrostatic interactions. These interactions should give an average 
distance between inorganic networks within the bilayer on the order of 10 A or less. 
Finally, if the dipping cycled is repeated through 150 cycles (FeNi-150), each bilayer of 
the Fe-CN-Ni network will be deposited onto the previous bilayer forming a y-type LB 
film with alternating regions of organic-to-organic and inorganic-to-inorganic contacts. 

DC Magnetometry 

The field-cooled (M fc ) and zero-field-cooled (H*) magnetizations as a function 
of temperature for FeNi-150 in two orientations with respected to the applied field are 
shown in Figure 6-1 . Both M (c curves show a rapid rise in the magnetization at lower 
temperatures below approximately 10 K, gradually saturating below 5 K. This behavior 
is indicative of ferromagnetic exchange in the film. Ferromagnetism is rationalized in 






3 r-r 



-2 - 



107 



I ' ' ■ ■ I ■ ■ ■ ■ I ■ ■ ' ■ I ' ' ■ ' I 




i i i i I i i i i I 



-L 



-750 -500 -250 







H(G) 



-L 



250 500 750 



Figure 6-2. Background corrected M(H) measured at 2 K for FeNi-150 aligned parallel 
(o) to the applied field and perpendicular (V) to the applied field. (The total sweep width 
was -50 kG to 50 kG). The lines are guides to the eye. 



this system as resulting from the orthogonal low spin S = Vi Fe m Tj g and S = 1 Ni 11 e g 
magnetic orbitals favoring the maximum total spin in accordance with Hund's rule. The 
field-cooled magnetic response of the material is clearly anisotropic with M,/ ~ 4Mj_ at 
T = 4 K and reflects the planar anisotropy of the network. 

Ferromagnetism in FeNi-150 is also supported by the hysteresis in M vs. H at 
T = 2 K as shown in Figure 6-2. Again, the magnetization is anisotropic with a larger 
remnant magnetization when the sample is aligned parallel to the applied field. The 
coercive fields are slightly anisotropic as well with He = 135 ± 5 G in the parallel 
orientation, and He = 1 10 ± 5 G in the orientation perpendicular. The nature of the LB 
film samples results in significant diamagnetic background arising from the substrate and 
sample container, making it difficult to quantify the film response at high magnetic fields. 
Therefore, the saturation moment and saturation fields are not well defined. 



108 





6 




i i i 1 


- r- 


i i i | i i i i 










H = H = 20 G " 

IC inc. is 


o 

3 


4 


— 


• 
• 


O 





'© 

2 


2 




• 
• 














• 


A 


A A n 






i 


• 

■ i i I 




... 1 ... . 



5 10 

T(K) 



15 



Figure 6-3. Background corrected M(7) for FeNi-bi aligned parallel to the applied field 
[(o) field-cooled and (•) zero field-cooled] and perpendicular to the applied field [(a) 
field-cooled and (A) zero-field cooled]. Cooling fields and measuring fields were each 
20 G. 



Plots of the magnetization of FeNi-bi as a function of temperature for both field- 
cooled and zero-field-cooling are shown in Figure 6-3 . Data are shown for the parallel 
and perpendicular orientations. As observed in the multilayer sample, there is a sudden 
rise in the field-cooled magnetization below 10 K, signaling ferromagnetic exchange. 
The magnetization tends toward saturation below 4 K. The field-cooled magnetic 
response shows a slightly larger anisotropy than observed in the multilayer sample with 
H/ ~ 5M X at T = 2 K. 

The M vs. H plots are shown in Figure 6-4 for FeNi-bi in both orientations. The 
magnetic behavior is anisotropic in both the remnant magnetization and coercive field 
with He = 75 ± 5 G in the parallel orientation, and He = 55 ± 5 G when perpendicular. 






3 i-t 



109 



i i I i i i i I i i i 



i i i i r 



■ ■ ■ ' * * * ■ ' 



-750 -500 -250 



I i i i i I i i i i 




_L 




H(G) 



J_i_ 



250 500 



750 



Figure 6-4. Background corrected M(H) at 2 K for FeNi-bi aligned parallel (o) to the 
applied field and perpendicular (v) to the applied field. (The total sweep width was -50 
kG to 50 kG). The lines are guides to the eye. 



The coercive fields for the FeNi-bi are significantly smaller than those observed in the 
multilayer film. 

The field-cooled and zero-field-cooled magnetizations as a function of 
temperature for the FeNi-mono sample are shown in Figure 6-5. The magnetic response 
in the monolayer film shows a higher anisotropy than the bilayer or multilayer films, with 
the magnetization in the parallel orientation an order of magnitude more intense than the 
perpendicular. The parallel M fc increases abruptly with decreasing temperature below 
T « 7 K, suggesting the onset of ferromagnetic order. The onset temperature is below the 
10 K seen in the FeNi-bi and FeNi-150 samples, perhaps reflecting the decrease in the 
number of possible exchange pathways in the isolated monolayer film. 



o 

3 



o 
C 1 



- 



110 



T — ' — "- 



KL. - M = 20 G 

PC mea* 




« 



a a I ft m ft * 



-i i i ■_ 



1 



A ft J 



5 10 

T(K) 



15 



Figure 6-5. Background corrected M(7) for FeNi-mono aligned parallel to the applied 
field [(o) field-cooled and (•) zero field-cooled] and perpendicular to the applied field 
[(a) field-cooled and (A) zero-field cooled]. Cooling fields and measuring fields were 
each 20 G. 




Figure 6-6. Background corrected M(H) measured at 2 K for FeNi-mono aligned 
parallel to the applied field. (The total sweep width was -50 kG to 50 kG). The lines are 
guides to the eye. 






Ill 

The M vs. H hysteresis loop for the FeNi-mono measured in the parallel 
orientation at T = 2K is shown in Figure 6-6. The data indicate a very weak hysteresis 
with a coercive field on the order of He = 10 G. This value is close to the instrumental 
resolution, which is limited by the pinned flux within the instrument, also found to be on 
the order of 10 G. 
AC Magnetometry 

The nature of the low temperature magnetic behaviors of the films were further 
probed using AC magnetometry. The temperature-dependent Xacfor FeNi-150 in the 
parallel orientation, measured at 17 Hz, 170 Hz, and 1.0 kHz are shown in Figure 6-7. 
The data show both %' (real) and x" (imaginary) components. The presence of a x" 
component is indicative of uncompensated moments in agreement with the hysteresis 
observed in the M(H) data, shown in Figure 6-2. The frequency dependence of the peak 
position in both components is a signature of spin-glass behavior. 126 The spin glass 
transition temperature, T g = 5.4 K, is defined by the maximum in the %\T) plot at low 
frequency, here 17 Hz. The anisotropy of the magnetization is again revealed by 
comparison to the Xac(7) data taken on FeNi-150 in the perpendicular orientation shown 
in Figure 6-8. While the two components are present with similar frequency dependence, 
the magnitude of the magnetic response is an order of magnitude lower than in the 
parallel orientation. The temperature of the % (7) maximum (5.2 K) is slightly less than 
that extracted from the parallel orientation. 

The Xac(7) data for the parallel orientation of FeNi-bi, shown in Figure 6-9, are 
noticeably different than what is observed for the multi-layer film. The broad feature in 
the %\T) is clearly resolved into two components in the x"(7) data. The presence of a 



112 




T(K) 



Figure 6-7. The background corrected AC susceptibility, %\T), [(o) 17 Hz, (A) 170 Hz, 
and (0)1 kHz] and%"(T), [(•) 17 Hz, (a) 170 Hz, and (♦) 1 kHz] for FeNi-150 aligned 
parallel to the applied field. The samples were measured with an AC field of 4 G under 
zero applied DC field. 



0.50 



e 



< 



0.25 - 



0.00 



o 




°A x' 


■ 


OA ^xr , 
OA <§, 


■ 


oao e 

A* 


- 


o% 6 


■ 


oA> O 




°& V" *6 


■ 


• ^ ■ V 


■ 


*?*<>$«o, 


— i — i — i — i — i — i — i — i — i — 1_ i i • i 



5 10 

T(K) 



15 



Figure 6-8. The background corrected AC susceptibility, x\T), [(°) 17 Hz, (A) 170 Hz, 
and (0)1 kHz] and x"(7), [(•) 17 Hz, (a) 170 Hz, and (♦) 1 kHz] for FeNi-150 aligned 
perpendicular to the measuring field. The samples were measured with an AC field of 4 
G under zero applied DC field. 



113 







■ ■ i ■ i > 1 1 ■ ■ i ■ 




^». 


. 


1.0 


■ 6*S 1' 


- 




: m f\'' ' 




3" 

S 
D 

h ° 5 




■ 


< 


. o*> \ * 




■K 


+ %* 




0.0 


• 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 


■ i i i i i i i i i i i- 



6 8 
T(K) 



10 



12 14 



Figure 6-9. The background corrected AC susceptibility, yC(T), [(■) 1Hz, (o) 17 Hz, (A) 
170 Hz, and (0)1 kHz] and x"(T), [(a) 1 Hz and (•) 17 Hz] for FeNi-bi aligned parallel 
to the measuring field. The samples were measured with an AC field of 4 G under zero 
applied DC field. 



X"(7) component is evidence of an uncompensated moment, which further supports the 
hysteresis observed at T = 2 K in the M(7) data, Figure 6-4. Spin glass behavior in FeNi- 
bi is indicated by the frequency dependence of the x'(7) and %"(T) peak positions. The 
peak position of the higher temperature shoulder in the %'(T) is difficult to discern 
unambiguously, but is very similar to the peak at T g = 5.4 K observed for the FeNi-150 
sample at the same frequency (17 Hz). The data for the two samples are compared in 
Figure 6-10. The lower temperature peak of FeNi-bi yields a separate glass temperature 
at 17 Hz of T g = 3.8 K. An interpretation of this feature will be discussed below. The 
anisotropy of the DC measurements is reproduced in the Xac(7) where the perpendicular 
susceptibility (not shown) is an order of magnitude less intense than that seen in the 
parallel orientation. 









5.0 



. (x 100) 
\ 



§ 2.5 

c 



i 



0.0 - 



114 



T 1— I 1 1 1 1 I 



fi 









J ^* 



tUtUit* 



10 



15 



T(K) 



F/gwre 10. The background corrected AC susceptibility, i\T), measured in 1 7 Hz for 
FeNi-bi (A) (scaled x 100) and FeNi-150 (•). Both samples were aligned parallel to the 
measuring field. The samples were measured with an AC field of 4 G under zero applied 
DC field. 



The Xac(7) data measured at 1 Hz, 17 Hz, 170 Hz, and 1.0 kHz for the parallel 
orientation of FeNi-mono are shown in Figure 6-11. The presence of a yC(T) component 
indicates the presence of an uncompensated moment in the monolayer film, albeit the 
peak has not clearly formed by T = 2 K, and lends support to the weak hysteresis 
observed in the DC M(//) data at T = 2 K. The effect of frequency on the x"(T) peak 
position cannot be unambiguously determined, however a shift in the onset temperature is 
suggested in the data. The %\T) clearly shows a frequency dependence which again is 
indicative of spin-glass behavior. The glass temperature T g = 2.4 K extracted from the 
peak position at 17 Hz is below the glass temperatures of the bilayer and multilayer films. 
Comparing the magnetic response of the film between two orientations shows a similar 
magnetic anisotropy to that observed in the x<k measurements with Xac an order of 



115 



5.0 



| 2.5 



* s 



1 1 1 1 1 

■ 4 


■ 


■ 
J 1 ■ 1 


- /of 

■feA 




■ 


■ ■III 


*A 


A 




, °. 1 . . O . 



10 



15 



T(K) 



Figure 11. The background corrected AC susceptibility, x'(7), [(■) 1Hz, (o) 17 Hz, (A) 
170 Hz, and (0)1 kHz] and yC\T), [(□) 1 Hz and (•) 17 Hz] for FeNi-mono aligned 
parallel to the measuring field. The samples were measured with an AC field of 4 G 
under zero applied DC field. 



magnitude larger in the parallel orientation versus the perpendicular orientation (not 
shown). 

Discussion 
Magnetic Anisotropy 

The magnetic anisotropy of the thin film samples containing the Fe-CN-Ni grid 
networks provides convincing evidence for the low-dimensional nature of the materials. 
Even though a crystal-like analysis is not possible, some general conclusions can be made 
regarding the nature of the anisotropy. The magnetic easy axis clearly has a major vector 
component oriented parallel to the plane of the substrate surface, i.e. within the plane of 
the grid network. This anisotropy most likely arises from the spin-orbit coupling of the 
magnetic moments to the structural anisotropy. The most pronounced anisotropic 
behavior is observed in the monolayer sample, FeNi-mono, which has a ratio of field- 






116 

cooled M : Mx at T = 2 K of ca. 22:1. This ratio decreases in the bilayer and 150 bilayer 
samples to ca. 4.6:1 and 3.4:1, respectively. This trend suggests the presence of 
interlayer coupling between the face-to-face networks within the bilayer region. This 
exchange is also evidenced by the increase in T c from 2.4 K in the monolayer film to 
5.4 K in the bilayer and multilayer films. 

The nature of the bonding interactions between adjacent networks within the 
bilayer regions remains uncertain. However, a comparison of the magnetic behavior of 
the bilayer and multilayer samples to those of other 2D metal-cyanide systems may 
provide some insight. Of particular relevance are the quasi-2D square-grid 37 " 40 networks 
constructed from iron(III)hexacyanide and nickel bis( diamine) complexes and the 
honeycomb 41 " 43 ' 210 ' 211 networks constructed from chromium(III)hexacyanide and 
nickel(cyclam) complexes. In general, these lamellar solids consist of cyanide-bridged 
two-dimensional sheets separated one from another by a solvent or counterion layer. The 
exchange within the sheets is ferromagnetic, but when the inter-sheet separation is 
reduced below ca. 10 A, inter-sheet antiferromagnetic exchange often results leading to 
an antiferromagnetic ground state 43 - 196 - 210 - 211 . 213 - 214 g ven though the inter-planar distance 
within the bilayers of the FeNi-bi and FeNi-150 samples are less than 10 A as evidenced 
from x-ray diffraction data, 205 no metamagnetic behavior is observed in these samples. 
This suggests that the interaction between face-to-face networks within the bilayer region 
is predominantly covalent in nature, brought about through bridging of the axial cyanide 
of the amphiphilic pentacyanoferrate complex to available coordination sites of nickel 
ions in the adjacent network. This covalent bonding arrangement would favor 



117 

ferromagnetic exchange by the same mechanism that promotes ferromagnetism within 
one network plane. 
Spin Glass Behavior 

The DC magnet ometry studies on each of the three samples yielded Mf C and Mzf c 

traces (Figures 6-1, 6-3, and 6-5) with a characteristic "A," shape that is typically observed 
in either ferromagnetic or spin glass materials. 126 ' 218 The films also displayed a hysteresis 
in their magnetization vs. field plots that are also signs of ferromagnetic order or spin 
glass behavior. On the other hand, the frequency dependence of the Xac(7) data for each 
of the three samples investigated indicates that these materials are not long-range ordered 
ferromagnets at low temperature, since such systems would not show a frequency 
dependence in their susceptibility at or below 1 kHz. Such frequency dependence is 
typically assigned to either superparamagnets or spin glasses. A spin glass state can be 
distinguished from a superparamagnet by quantifying the frequency dependence through 
the ratio (f> ne 

<|> = A7'f/[(7fA(logH)))] (1) 

where T? is the temperature at which the maximum in yC(T) occurs, and AFf is the 
difference in Tt between an initial frequency © and final frequency <x>f. The values of <fi 
obtained for FeNi-bi and FeNi-150 are 0.05 and 0.04, respectively, which fall within the 
typical range for insulating spin-glasses and are very similar to those reported by 
Buschmann 216 " 217 for a series of hexacyanomanganate Prussian blue analogues. 

A somewhat larger value of <j>= 0. 10 was obtained for FeNi-mono which falls 
between the extreme case of <p= 0.28 reported for the superparamagnet a-(Ho2C»3)(B203), 



118 

and (j) ~ 0.005 for insulating spin-glasses. I26 Attemps to fit the data to the Arhenius 
equation, eq. 2, 

ln(x/x ) = [E a /(k B T)] (2) 

where x is the average relaxation time corresponding to the frequency of the AC 
measurement, and E a /ke is the energy barrier to magnetic reversal in an isolated particle, 
yields a x = 1 x 10" 14 sec and Ea/ke = 70 K. The value of x = 1 x 10" 14 sec fall below the 
range of 1 x 10" 11 < x c < 1 x 10" 9 predicted for non-interacting ferromagnetic particles 219 
and indicates the presence of inter-particle interactions. The strength of the interaction 
increases significantly as one progresses to the isolated bilayer and multilayer films 
where x = 1 x 10" 20 sec (E a /k B = 170 K) and x = 1 x 10" 30 sec (E a /k B - 350 K), 
respectively. Therefore, the system may best be described as a progression from 
moderately interacting ferromagnetic particles in the monolayer to a collective strongly 
interacting glass-like state in the bilayer and multilayer films. 

The two peak profile observed in the %{T) data for FeNi-bi is unique to the 
sample and warrants further discussion. Similar dual peak profiles observed in 
M[Mn(CN) 6 ] (M = Cr, Mn) Prussian blue analogues have been assigned to reentrant spin 
glass behavior, however such a characterization here is inappropriate since both peaks 
show frequency dependence. Therefore neither peak corresponds to an ordered 
ferromagnetic state. A more likely explanation arises by comparing the xJiJ) data for 
FeNi-bi to the iJJ) data for FeNi-150, as shown in Figure 6-10. The glass transition 
temperature is a function of the disorder of a material, with more disordered materials 
giving lower glass transition temperatures. Analysis by grazing incidence diffraction 
indicates in-plane coherence lengths of about 6 unit cells. This estimate is an average, 



119 

and each bilayer will have some domains that are larger and some smaller, and some 
areas that are relatively disordered. The two processes observed in Xac(7) likely reflect 
two different sets of domains. Since the bilayers are fabricated using the same process 
for each sample, the FeNi-bi and FeNi-150 should possess essentially the same degree of 
structural disorder within each bilayer. Therefore the absence of a low temperature peak 
in the FeNi-150 sample may be due to interactions that are present in the multilayer 
sample but absent in the single bilayer, such as long-distance dipolar exchange forces. 
The dipolar exchange between layers could provide an extra interaction pathway that ties 
the less coherent two-dimensional domains to the larger ones. The lack of similar dipolar 
forces in the isolated bilayer sample results in each of these domains adopting a unique 
glass transition temperature. Similar dipolar forces have been shown to operate over 
relatively long distances (-35 A inter-bilayer spacing) in other lamellar ferromagnetic 
materials. 44 

Conclusions 

The interfacial assembly of a two-dimensional Fe-CN-Ni grid network in 
combination with the controlled deposition capability of the Langmuir-Blodgett 
technique yields novel low-dimensional thin films where the effects of interlayer 
interactions on magnetic properties can systematically be studied. The results show a 
variation in magnetic behavior upon progressing from a two-dimensional monolayer to a 
two-dimensional bilayer to a multilayer film. The progressive changes demonstrate the 
influences of dimensionality and interlayer coupling on the magnetic behavior in 
molecule-based materials. All of the materials studied show highly anisotropic magnetic 
behavior, with the highest anisotropy observed in the isolated monolayer film. Each 



120 

system also shows the presence of ferromagnetic domains at low temperature. The 
bilayer and multilayer assemblies show clear spin-glass behavior whereas the isolated 
monolayer film may be just as well described as a moderately interacting 
superparamagnetic system. 



CHAPTER 7 
SEQUENTIAL ASSEMBLY OF HOMOGENEOUS MAGNETIC PRUSSIAN BLUE 

FILMS ON TEMPLATED SURFACES 



Introduction 

Prussian blue is a mixed-valent cubic iron-cyanide polymer of approximate 
composition Fe(III)4[Fe(II)(CN) 6 ]3 14H 2 0. 191 The material possesses an intense metal-to- 
metal charge transfer absorbance band that gives it a deep blue color. The intense color 
of the material and the dependence of the color on oxidation state have led to numerous 
investigations into applications involving electrochromic materials. 220 " 223 In addition, the 
structure of the material typically includes defects due to incomplete displacement of 
coordinated water on the bridged metal sites. These defects lead to a charge imbalance 
that is compensated for by the incorporation of alkali cations into the structure. 195 As 
such, Prussian blue and related compounds have also been investigated for potential 
applications in cation sensors. 224 " 227 

The Prussian blue structure is not limited to iron and the compositions can be 
varied to include numerous combinations of transition metal ions in various oxidations 
states. The incorporation of various mixtures of transition metal ions and hexacyanides 
provides an opportunity to tune the physical properties of the resulting solids. One 
particular property of these Prussian blue "hybrids" that has generated a great deal of 
interest is their magnetic behavior. 13 ' 195 The synthetic versatility of Prussian blues 
coupled with the ability of the bridging cyanide ligand to efficiently mediate magnetic 



121 



122 

exchange has led to several high Tc mixed-spin molecular based magnetic 
systems, 52 ' 192 ' 22 '* including some which order well above room temperature. 51 ' 173 ' 174,215 
Also of interest is the photo-induced magnetization in Fe-Co Prussian blue 
analogues. 207 ' 229 " 231 

Much of the excitement over the magnetic behavior of the Prussian blue family of 
materials stems from the fact that they are a molecular-based class of materials. This can 
be viewed as an advantage over metal oxides since metal cyanides can be synthesized 
under mild synthetic conditions and easily isolated. Unlike oxides, which are typically 
black, most metal cyanides are colored species and thereby posses the potential to form 
transparent magnetic films. 56 

Many of the aforementioned properties of Prussian blues make them attractive 
candidates for uses in materials spanning a wide range of potential applications. Most of 
these applications would require incorporating these functionally diverse materials into 
homogeneous thin films, and several methods aimed at depositing Prussian blue analogs 
onto solid supports have been reported. These methods include adsorption on sol-gel 
films, sequential deposition onto polyelectrolyte coated surfaces, 224 adsorption at 
Langmuir-Blodgett films. 64 ' 233 The most commonly applied technique is the 
electrochemical deposition of the materials on electrode surfaces, 54 ' 56 ' 227 ' 234 " 239 The 
morphologies of the respective films vary from disperse collections of crystallites to 
fairly continuous amorphous coatings. 

Our approach to forming thin homogeneous Prussian blue films is outlined in 
Scheme 7-1. The method is based on the sequential deposition procedure that has 
previously been used with polyelectrolyte-modified surfaces. However, our method 



123 



CHj CHj 

(CHj^CH,),, 
V 



6 






1 

(Langmuir 
monolayer) 



+ 



OH, 

h,<k | jtm, 

H,0^ | ^OH, 
OH, 



Square Grid 
Network 



A 




(subphase) 



Air/water Interface 




Scheme 7-1. Sequential deposition of Prussian blue begins by first templating a 
hydrophobic surface with one monolayer of a two-dimensional Fe-CN-Ni network 
preformed at the air-water interface (A). The monolayer film is transferred by the LB 
technique and removed from the trough after a single transfer (once down). The 
templated surface now consists of what is essentially one layer of the Prussian blue 
structure (B). Subsequent immersion in an aqueous solution of the appropriate metal ion 
(C) followed by immersion in an aqueous solution of a hexacyanometalate (D) results in 
the growth of a thin film of the polymeric metal cyanide. The cycle is repeated as 
necessary (typically 5-10 cycles) to achieve a homogeneous film (E). 



124 

differs in that the hydrophobic surface is first templated with a two-dimensional iron- 
nickel-cyanide grid network. The template layer is prepared at the air-water interface of 
a Langmuir-Blodgett trough by spreading a monolayer of an amphiphilic 
pentacyanoferrate(III) complex on a subphase containing aqueous nickel (II) ions. The 
two-dimensional grid network which forms has been well characterized and is, in 
essence, one layer of the cubic Prussian blue cell. 205 The reacted film can then be 
transferred onto a hydrophobic support by the Langmuir-Blodgett technique. The film is 
only transferred in the down direction leaving the surface of the substrate terminated with 
the reactive cyanides from the pentacyanoferrate complex which are ideally organized 
into a 2D face-centered grid motif. As such, the surface is well matched for subsequent 
epitaxial deposition of the bulk cubic solid via the sequential adsorption of first aqueous 
metal ions, then hexacyano metal ions. Surface morphology investigations using AFM 
and SEM indicate that the surface coverage is exceptional, and magnetic measurements 
reveal that the magnetic properties of the solid-state materials are reproduced in the thin 
film structures. 

Experimental 
Materials. Reagent grade FeCl 2 , Ni(N0 3 ) 2 6H 2 0, CrCl 2 , K 3 Fe(CN) 6 , K 3 Cr(CN) 6 , 
Ag(N0 3 ), and Csl were purchased from Aldrich (Milwaukee, WI) and used without 
further purification. The Cs x Kd. x) [Cr(CN) 6 ] complex was prepared as follows. To 25 
mL of an aqueous solution containing lg (3.1 mmol) K 3 Cr(CN>6 was added 25 mL of an 
aqueous solution containing 3.3 equivalents (10.0 mmol) of Ag(N0 3 ). The precipitated 
AgsCr(CN)6 was collected by filtration, washed thoroughly with water, and suspended in 
a 20 mL aqueous solution of 3.2 g (12.4 mmol) Csl. The solution was stirred vigorously 



125 

at room temperature for 2 hours and the Agl that formed was removed by filtration 
through Celite. Addition of 50 mL absolute ethanol to the filtrate precipitated the 
Cs3[Cr(CN)6] complex salt, which was collected by filtration and subsequently dried 
under vacuum in a desicator (yield 1 .4 g). The complex gave a UV-Vis and IR spectra 
identical to the K3Cr(CN)6 starting material. The amphiphilic complex 
bis(tetramethylammonioum) pentacyano(4-didodecylaminopyridine)ferrate(III) 6H2O (1) 
was prepared as previously described. 205 

Instrumentation. UV-Vis spectra were obtained on a Hewlett-Packard 8452A 
diode array spectrophotometer. The LB films (template monolayers) were prepared by 
using a KSV Instruments 5000 trough modified to operate with double barriers. The 
surface pressure was measured with a filter paper Wilhelmy plate suspended from a KSV 
microbalance. Subphase solutions were prepared from 17.8-18.1 MQ cm water delivered 
with a Barnstead E-pure system. Magnetization measurements were performed on a 
Quantum Design MPMS SQUID magnetometer. Grazing incidence x-ray diffraction 
(GIXD) experiments using synchrotron radiation were performed at the Advanced Photon 
Source, Argonne, IL, at the Materials Research Collaborative Access Team beamline 
(sector 10). The GIXD scans were performed on films prepared on glass slides. The 
samples were positioned in the center of an 8-circle Huber goniometer and oriented at an 
angle of 0. 13 degrees relative to the incident beam. The incident beam was collimated to 
200 microns high by 1500 microns wide and tuned to a wavelength of 1.254 A. 
Diffracted intensity in the xy plane was measured using a Nal scintillation counter 
mounted on the Huber goniometer. The diffracted signal was collimated prior to the 
detector using Soller slits giving an experimental resolution on the order of 0.015 A' 1 . 



126 

Film preparation. AFM, SEM, UV-Vis, and GIXD samples were prepared on 
glass slides that were purchased from Buehler Ltd (Lake Bluff, IL). Samples for SQUID 
investigations were prepared on Mylar (Dupont) substrates cleaned prior to use with 
absolute ethanol. The glass substrates were cleaned using the RCA procedure 178 and 
made hydrophobic by deposition of a monolayer of OTS. 179 ' 180 

Formation of the template layer. The amphiphilic iron complex 1 was spread 
from a chloroform solution onto surface of a subphase 1 g/L in Ni(NC>3) 2 6H 2 0. All 
template monolayer films of the nickel-iron-cyanide network were transferred with one 
downstroke onto hydrophobic substrates at a surface pressure of 25 mN/m. The average 
transfer ratios were >90%. 

Formation of the Prussian blue films. The bulk Prussian blue film is assembled 
onto the templated substrate by a sequential deposition process. After transfer of the 
template layer by the LB method, the substrate was removed from the trough and rinsed 
briefly with water. The substrate was then immersed in the appropriate 0.01 M aqueous 
FeCl 2 , Ni(NC>3)2 6H2O or CrCl 2 solution for ~ 1 min then rinsed twice by brief immersion 
in two separate beakers of nanopure water, then once by immersion in methanol, and 
finally dried under a stream of nitrogen before the process was repeated with an aqueous 
solution 0.01 M in the appropriate K 3 Fe(CN) 6 or K 3 Cr(CN) 6 complex salt. The 
K x Cr n (i. x) [Cr ni (CN)6] film was prepared under a N 2 atmosphere with N 2 -purged solutions. 
A Cs x K(i. X )[Cr(CN)6] solution 10 mM in CsN0 3 and was used in the synthesis of the 
Cs x Ni (i. X )[Cr m (CN) 6 ] film. One deposition cycle is comprised of one immersion, 
subsequently, in each of the metal ion solutions. 



127 

Results and Discussion 
Film Deposition 

Our method for preparing thin metal cyanide films is outlined in Scheme 7-1. 
The schematic is specific to the preparation of a Prussian blue film, but the method itself 
is general and various combinations of aqueous metal salts and hexacyano complexes can 
be substituted for the Fe ( aq > and [Fe(CN)6]' " ions depicted in Scheme 7-1 . Central to the 
concept is the preparation of the template layer that is formed by the reaction of a 
Langmuir monolayer of an amphiphilic pentacyanoferrate(III) complex (1) with aqueous 
Ni ions in the subphase. Nickel ions are used in the template layer since these ions 
have been shown to form a structurally coherent two-dimensional grid network when 
reacted with l. 205 Transfer of the grid network to a hydrophobic surface by the Langmuir- 
Blodgett technique is terminated with the substrate immerged in the subphase and 
subsequently removed, then cycled through solutions of the appropriate metal complex 
building blocks. In this way, a thin film of the bulk solid is deposited on the surface. 

Two AFM images, typical of the films prepared by Scheme 7-1, are shown in 
Figure la and lb. Figure la is an AFM image of a Ni[Cr in (CN) 6 ] film after 3 deposition 
cycles, and Figure lb is an AFM image of an Fe in [Fe n (CN) 6 ] film after 5 deposition 
cycles. Both films were prepared on glass substrates made hydrophobic by a monolayer 
of octadecyltrichlorosilane. Both images show complete surface coverage over the 
100 pm area investigated. The darker brown background in both images was verified as 
the metal-cyanide film by comparison to other images taken at defect sights. The lighter 
regions in both films are due to smaller crystallites on the surface and indicate that film 
growth likely proceeds through domain formations and not via a layer per cycle 
mechanism. This conclusion is also supported by the ~ 30 nm average thickness of the 



128 





Figure 7-1. AFM images of (a) a Ni[Cr m (CN) 6 ] film after 3 deposition cycles, and (b) 
an Fe m [Fe n (CN)6] film after 5 deposition cycles. Both films were prepared on glass 
substrates as described in the text. 




■■.■■:■:■::■■ 

mm 

v;-mv 



wmmm 



043962 



kV X 1 . 5 K 2 . 8 *■ m 



Figure 7-2. An SEM image taken of an Fe ni [Fe n (CN)6] film after 10 cycles. The film 
was prepared on a glass substrate as described in the text. 



129 




M 



II 



. « v X30. 3K i . ee» 



i V X30 . 0K 1 . 00. 



Figure 7-3. SEM images of (A) an Fe in [Fe n (CN)6] film after 10 cycles and (B) a 
Ni[Fe (CN) 6 ] film after 10 deposition cycles. Both films were prepared on glass 
substrates as described in the text. 



films, which is higher than, would be expected for cubic particles with a 1 nm unit cell 
length. Further evidence that the surface coverage is complete in these films is provided 
by the SEM images taken of the materials. An SEM image taken of an Fe III [Fe I1 (CN) 6 ] 
film after 10 cycles is shown in Figure 7-2. The abrasion in the upper left corner clearly 
shows the substrate below a Prussian blue film that is continuous over the remaining 
400 urn 2 . Similar results were obtained on the other metal-cyanide films. Images taken 
at higher magnification of the same Fe ni [Fe n (CN) 6 ] film and of a different Ni[Fe HI (CN) 6 ] 
film after 10 deposition cycles are shown in Figures 7-3a and b, respectively. The images 
are similar to those obtained with AFM (Figure 7-1) and show the presence of smaller 
crystallites above a continuous underlying film. 

The surface morphology studies discussed above show that the assembly process 
depicted in Scheme 7-1 is somewhat idealized with respect to the mechanism of film 
growth. The film is not deposited as one unique layer per cycle (one cycle being steps C 



0.75 



o 



0.50 









o 

on 



0.25 



0.00 










130 



| i I I 



J 1 ' 



25 50 75 

Number of depositions 



100 



Figure 7-4. Linear fit to the absorbance at 730 nm versus deposition cycle for the 
FeIII[FeII(CN) 6 ] film through 100 cycles. The film was prepared on a glass substrate as 
described in the text. 



and D) but through a mixture of domain formation and multiple deposits per cycle that 
lead to a complete surface coverage, albeit with variations in film thickness. Even so, the 
growth of the film is quite regular in quantity as evidenced by the near linear increase in 
absorbance versus deposition cycle for the Fe in [Fe n (CN)6] film through 100 cycles, as 
indicated in Figure 7-4. Furthermore, the transparency of the resulting blue film provides 
further evidence of structural homogeneity and small grain size in the material 
(Figure 7-5). 

Structural characterization of the self-assembled films was performed using 
grazing incidence x-ray diffraction from a synchrotron source. 205 The diffraction patterns 
obtained for a Fe ni [Fe n (CN) 6 ] film after 100 deposition cycles and a Ni n [Cr m (CN) 6 ] film 



131 




Figure 7-5. A photograph demonstrating the transparency of an Fe m [Fe n (CN)6] film 
after 100 deposition cycles. The film was prepared on a glass substrate as described in 
the text. 



after 20 cycles are shown in Figure 7-6 and Figure 7-7, respectively. Both diffraction 
patterns can be indexed to a face-centered cubic cell with a = 10.26 A for the 
Fe UI [Fe n (CN)6] structure, and a = 10.48 A for the Ni n [Cr in (CN) 6 ] structure. Both values 
of the unit cell parameter are within the range typically observed for Prussian blues 
compounds. 13 > 195 - 209 ' 234 Analysis of the (200) peak widths in each of the diffraction 
patterns by application of the Scherrer method 147 yielded average structural coherence 
lengths of 190 A and 150 A for the Fe ni [Fe n (CN)6] and Ni^Ct^CNfe] films, 
respectively. 



132 






o 
U 







(200) 






800 




+ 
+ 




• 






+ 
+ 


(220) 


■ 


400 
n 




(400) " 
% (420). 

i . . 1 



d spacing (A) 



Figure 7-6 The GIXD pattern obtained for an Fe in [Fe n (CN)6] film after 100 deposition 
cycles. The Miller indices are derived from a cubic unit cell with a = 10.26 A. 



250 



g 125 

3 
O 

O 



(200) 


i i i i i i i i 


+ 




+ 




(220) 




% 


■ 


+ + 


(400) 


+* 


* 


++ 








>r : #ir^ 


$F^*^ . 


+ 

i i i i i i ■ i i ■ i i i i i i 





5 4 3 

d spacing (A) 



Figure 7-7. The GIXD pattern obtained for an Ni II [Cr III (CN) 6 ] film after 20 deposition 
cycles. The Miller indices are derived from a cubic unit cell with a = 10.48 A. 



0.02 



o 
i o.oi 





0.00 







133 



T — ' — "" 



04 
0.2 



® 0.0 

g-0.2 

-0.4 






-200 -100 100 200 
H(G) 



J i_ 



4 6 

T(K) 



10 



Figure 7-8. The field-cooled magnetization in 20 G as a function of temperature and 
hysteresis loop (inset) for a 10 cm 2 K x FeIII ( i. x) [FeII(CN) 6 ] film after 100 deposition 
cycles. 



Magnetism 

The magnetic properties for each of the films prepared on Mylar substrates were 
measured as a function of temperature and field. The results for the 10 cm 2 
K x Fe (i- x) [Fe (CN) 6 ] film measuring after 100 deposition cycles are shown in Figure 7-8. 
The solid-state analogue is a known ferromagnet below a T c = 5.6 K 240 and similar results 
are observed in the current thin film sample. The field-cooled magnetization versus 
temperature, M fc (7), shows a rapid rise at T c = 5 K and the material displays a clear 
hysteresis at T = 2 K in the magnetization versus field, M(//), plot with a coercive field 
(He) of 30 G (Figure 7-8 inset). Both behaviors are consistent with the presence of 
ferromagnetic order in film. 



134 



5.0 

4.0 

O 3.0 



2.0 



* 



1.0 



0.0 



-i — i — I — i — r- 



I I ' I I I 



-t — i — i — i — r 



1.0 
0.3 

S ! o.o 



. ' ■ ' 1 ' ' ' 
: T = 5K 

• 


■■■!•■', 


%**• • • 

' 


• 

... 1 ... ■ 



-200 -100 100 200 



••••••• • • • • 

J 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I I I I ■ ■ ■ I I ■ . . I 



25 50 75 100 125 150 

T(K) 

Figure 7-9. The field-cooled magnetization in 20 G as a function of temperature and 
hysteresis loop (inset) for an 8 cm 2 Cs x Ni ( i. x) [CrIII(CN)6] film after 20 deposition cycles. 



The M fc (7) and M(H) data for a 8 cm 2 C^Ni^.^Cr^CN^] film after 20 
deposition cycles is shown in Figure 7-9. The ferromagnetic ordering temperature, 
T c = 75 K, extracted from the M fc (7) data is slightly lower than the T c = 90 K reported for 
the bulk solid. 169 Ordering temperatures in Prussian blue analogues C x M A ( i. x) [M B (CN) 6 ] 
are known to be sensitive to the ratio of M A to M B and this ratio is often affected by the 
identity of the counterion C. 195 Since the structural coherence of the film was confirmed 
by GIXD, the lower ordering temperature observed in the film most likely reflects a 
lower Ni:Cr ratio in the film relative to the solid-state material. The material displays a 
clear hysteresis at 5 K with an He of 70 G and a remnant magnetization (M rem ) 50% of the 
saturation value. Both of these values are nearly identical to those reported by Gadet for 
the solid analogue. 195 



135 



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3 



2.0 



1.5 



1.0 



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0.0 



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-3-2-10 1 2 3 
H(kG) 









l l l I 







10 



15 20 
T(K) 



25 



30 



Figure 7-10. The field-cooled magnetization in 20 G as a function of temperature and 
hysteresis loop (inset) for a 6 cm 2 K x Ni n (i. X )[Fe ni (CN)6] film after 10 deposition cycles. 



Magnetic measurements were also undertaken on a 6 cm 2 K x Ni n ( i. X )[Fe in (CN)6] 
film after 10 deposition cycles. The results are shown in Figure 7-10. The M fc (7) data 
show a transition to a ferromagnetic state at T c = 1 8 K. This is lower than the T c = 23.6 
K reported for the analogous solid. 193 Once again, the lower T c in the film relative to the 
solid most likely results from a slightly lower Ni : Fe ratio in the material. 195 The 
hysteresis loop measured at T = 2 K for the material shows a M rem which is 30% of the 
saturation value and a relatively large He = 1000 G. The magnitude of the coercive field 
is less than half the He = 2500 G reported for the solid 193 and may reflect an improved 
structural coherence in the thin film sample. 

Higher T c films are obtained when the K x Cr I1 ( i. x) [Cr III (CN) 6 ] compound is 
assembled on the surface. The K x Cr I1 (1 . x) [Cr III (CN) 6 ] family of materials are known 



136 





1.6 




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..-.. 


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50 100 150 200 250 300 
T(K) 



Figure 7-11. The field-cooled magnetization in 20 G as a function of temperature and 
hysteresis loop (inset) for a 7 cm 2 K x Cr II (i. X )[Cr III (CN) 6 ] film after 40 deposition cycles. 



ferrimagnets with 150 K < T c < 270 K depending on the Cr" : Cr 111 ratio. 52 * 54,233 The 
higher T c 's result from the cyanide mediated antiferromagnetic coupling between 
adjacent metal sites. The Mf C (7) for a 7 cm 2 K x Cr II (i. X )[Cr III (CN)6] deposited film after 40 
deposition cycles is shown in Figure 7-11. The data show the onset of long-range order 
at a T c = 215 K which is well within the reported range for K x Cr II (i. X )[Cr III (CN)6] 
compounds. 52 ' 54 " 233 The hysteresis loop measured at 10 K showed a very weak He ~ 10 G. 
The small value of H c suggests a significant oxidation of the Cr 11 species in the film, as a 
higher Cr 111 content has been shown to greatly reduce the value of the coercive field in 
chromium cyanides. 2 3 



137 

Conclusions 

A method for the deposition of bulk Prussian blues as thin homogenous films 
using an LB deposited monolayer of an Fe-CN-Ni two-dimensional grid network 
preformed at the air-water interface as a template layer has been demonstrated. The 
procedure is applicable to hydrophobic surfaces and thus compliments the often-used 
electrochemical methods that take place at conducting surfaces. The technique yields 
films with virtually complete surface coverage as evidenced by AFM and SEM. 
Application of the synthetic procedure yielded magnetic films with properties comparable 
to their solid-state analogues. The results demonstrate a potential application of 
interfacially assembled inorganic networks as templates for the epitaxial growth of thin 
solid films. 



CHAPTER 8 
INVESTIGATIONS INTO THE INTERFACIAL ASSEMBLY OF LINEAR CHAIN 

AND 2D HEXAGONAL NETWORKS 



Introduction 

The linear cyanide bridge is a versatile building block in supermolecular 
chemistry. When used in the form of simple octahedral hexacyano complexes bridged 
through other octahedral complexes, the inherent 90 degree bond angles lead to the cubic 
Prussian blue structures. Additionally, the carbon and nitrogen end of the ligand have 
different ligand field strengths. This can result in the carbon-bound complex adopting a 
tetrahedral or square planar geometry whereas the nitrogen bound complex remains 
octahedral. The bridged networks then contain a mixture of site symmetries that 
subsequently result in networks other than simple cubic networks observed in the 
Prussian blue materials. This synthetic approach has been exploited in some cadmium 
cyanides where the clathration of solvent molecules leads to porous solids similar to 
zeolites. 241 " 243 Lower symmetry metal cyanides such as the square planar [Ni(CN)4] 2 " can 
also be bridged through octahedral complexes yielding planar networks. 244 " 246 

A different and far more versatile synthetic strategy employed in supermolecular 
cyanide-bridged systems is the use of "blocking ligands" on one or both of the complex 
building blocks. The blocking ligands effectively lower the symmetry of the system and 
can prevent the uninhibited growth of the polymer through three dimensions. Some of 



138 



139 

the blocking ligand that have been used to date include bidentate diammines and 
bipyridines, tridentate and tetradentate cyclic amines, and planar conjugated systems such 
as salen and terpyridine. The resulting networks vary from "zero"-dimensional clusters 
to linear chains to two-dimensional arrays. ' ' ' ' ' 

The successful preparation of square-grid networks through the interface directed 
assembly of an amphiphilic pentacyanoferrate complex with octahedral Ni 2+ , Co 2+ , and 
Mn 2+ ions as described in the previous chapters demonstrates the utility of an air- water 
interface as a structure directing entity in the formation of low dimensional inorganic 
solids. This same technique, when used in conjunction with complexes having various 
blocking ligands, may allow for the assembly of other networks with different structural 
motifs. With this goal in mind, two novel amphiphilic complexes possessing different 
symmetries were reacted at the air- water interface (Figure 8-1). The first complex 
discussed is an amphiphilic Fe(III) terpyridine complex that acts as a T-shaped building 
block. As shown in Scheme 8-1, the symmetry of the complex is well suited for the 
assembly of a linear chain polymer if reacted at the air- water interface with a linear 
bridging unit such as [Ag(CN)2] dissolved in the subphase. The feasibility of the 
approach has been demonstrated by Woodward et al who have recently reported the 
crystal structure of a Ni(terpyridine)Ni(CN) 4 linear chain polymer. 247 

The other system reported herein involves the reaction of an amphiphilic cyclic- 
ammine nickel(II) complex Ni(cyclam) with nickel tetracyanide and chromium(III) 
hexacyanide at the air-water interface. Similar reactions in solution with non-amphiphilic 
nickel cyclam complexes have resulted in linear chains ' with Ni(CN)4 " and two- 
dimensional "honeycomb" hexagonal networks with Cr(CN)6 3 " (Figure 8-2). 41-43 - 211 



140 





Fe(terp} ) 



Ni(cjclam) 



Figure 8-1. Structures of the amphiphilic complexes Fe(terpy) and Ni(cyclam). 




£&> 




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«**♦-* ii****^ *«*•'**• 

• •»!«* • « "•!»* » 



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B 



Figure 8-2. Reported structures for the product of reactions with Ni(cyclam) complexes. 
A) linear chain incorporating the Ni(CN) 4 2 " complex, (from reference 213) B) two 
dimensional honeycomb network incorporating Cr(CN) 6 3 ' (from reference 247). 



141 




Langmuir 
monolayer 



■^ 



T\- 



+ ,.^ 



S 



subphase 




targeted linear 
chain structure 



Scheme 8-1. Strategy for the formation of a linear chain polymer at the air-water 
interface. 



Experimental 
Instrumentation. The LB films were prepared by using a KSV Instruments 5000 
trough modified to operate with double barriers. The surface pressure was measured with 
a filter paper Wilhelmy plate suspended from a KSV microbalance. Subphase solutions 
were prepared from 17.8-18. 1 MQ cm water delivered with a Barnstead Epure system. 
Magnetization measurements were performed on a Quantum Design MPMS SQUID 
magnetometer. All NMR spectra were obtained on a Varian VXR-300 spectrometer. 
The characteristic solvent peaks were used as reference values. Elemental analyses were 
performed by the University of Florida Spectroscopic Services laboratory. Melting 
points were obtained on a Thomas-Hoover Capillary melting point apparatus and are 
uncorrected. The FT-IR spectra as KBr pellets (solids) or on silicon ATR crystals (films) 
were recorded on a Mattson Instruments (Madison, WI) Research Series- 1 FTIR 
spectrometer with a deuterated triglycine sulfate (DTGS) detector. Grazing incidence x- 
ray diffraction experiments were performed at the Materials Research Collaborative 



142 

Access Team (MRCAT) beamline (sector 10), Argonne National Laboratory, Argonne, 
IL. The experimental setup has been described previously. 146 ' 205 For the Langmuir 
monolayer diffraction experiments (multilayer LB films), a X = 1 .254 A x-ray beam of 
dimensions 200 urn (height) x 2000 urn (width) was made incident on the water surface 
(glass surface) at an angle of 1 .8 mrad. 

Film preparation. The multilayer LB films, 30 bilayers per side, were prepared 
on glass microscope slides made hydrophobic by a monolayer of OTS. One equivalent of 
octadecanol was mixed with the Fe(terpy) complex to aid in the transfer of the film over 
the silver cyanide subphase. The films were transferred at a surface pressure of 30 mN/m 
at room temperature. 

Synthesis. All starting materials were purchased from Aldrich (Milwaukee, WI) 
and used as received except for the 4-hydroxybenzaldehyde which was recrystallized 
from water prior to use. The amphiphilic nickel perchlorate complex containing the 
ligand [l-(propanoic acid, stearyl ester)-l,4,8,ll-tetraazacyclotertradecane] (Ni(cyclam)) 
was supplied by Dr. Christophe Mingotaud (Centre de Recherche Paul Pascal, PESSAC, 
France). The following procedure is adapted from the method reported by Constable for 
the synthesis of 4 , -phenyl-2,2'-6',2"-terpyridine. 249 The method of Constable involves the 
condensation reaction of benzaldehyde with two equivalents of 2-acetylpyridine under 
basic conditions to form a diketone intermediate, which can be further reacted with 
excess ammonium acetate to afford the final 4-phenylterpyridine. To form the alkylated 
4-phenylterpyridine, the benzaldehyde is replaced in the synthesis with 4-tetradecyloxy- 
benzaldehyde. 



143 

4-tetradecyloxybenzaldehyde. The 4-tetradecyloxybenzaldehyde was prepared 
from a solution containing 8.4 g (30 mmol) 1-bromotetradecane and 3.66 g (30 mmol) 
4-hydroxybenzaldehyde in dimethoxyethane under reflux over 4.5 g (excess) potassium 
carbonate for 12 hrs. The solvent was removed under reduced pressure and 200 mL 
water added to remaining mixture. The product was removed by extraction with 150 mL 
diethyl ether. After drying over anhydrous magnesium sulfate, the ether solution was 
added to 150 mL acetonitrile and concentrated under a stream of nitrogen. The 
precipitate was filtered and used without further purification [(yield 7.4 g crude) (H 1 
NMR(CDC1 3 ), ppm: 9.88 (s) 1H, 7.82 (d) 2H, 7.00 (d) 2H, 4.03 (t) 2H, 1.81 (m) 2H, 
1.46-1.26 (m) 24H, 0.88 (t) 3H]. 

4'-(4-tetradecyloxyphenyl)-2,2'-6\2"-terpyridine To 70 mL of an ethanolic 
solution containing 2.24 g (40 mmol) KOH and 5.4 g 4-tetradecyloxybenzaldehyde at 
35°C was added drop wise 4.6 g (38 mmol) 2-acetypyridine in 10 mL ethanol. The 
solution turned deep red after several minutes and the solution was left to stir for 24 hrs. 
The white precipitate that formed was filtered and washed with ethanol until the washing 
were colorless. The solid was purified by recrystallization from 100 mL 3: 1 EtOH:THF 
and dried under vacuum, (yield 5.4 g) The product at this stage is a diketone 
intermedite 49 (FT-IR 1695 cm" 1 ), which is converted to the terpyridine by refluxing 8 hrs 
in air in an 80:20 EtOH:THF solution containing 8 g (excess) ammonium acetate. The 
dark green solution, when cooled in the freezer, precipitates the crude product 
terpyridine. Recrystallization for EtOH/THF yields the pure yellow product [(yield 2.75 g 
mp 94-96°C, calc.(obs.) for C35H43N3O: %C 80.57 (80.42) %H 8.31 (8.54) %N 8.05 
(8.04); H 1 NMR(CDC1 3 ), ppm: 8.68 (m) 6H, 7.81 (m) 4H, 7.30 (m) 2H, 7.00 (m) 2H, 



144 

3.98 (t) 2H, 1.81 (m) 2H, 1.46-1.26 (m) 24H, 0.88 (t) 3H]. The product is best stored in 
the dark since discoloration occurs with prolonged exposure to light. 

(4'-(4-tetradecyloxyphenyl)-2,2'-6',2"-terpyridine)iron(III)trichloride 
(Fe(terpy)). The complex was prepared by addition of a 10 mL THF solution containing 
650 mg (lmmol) to a 10 mL THF solution containing 850 mg (5 mmol) anhydrous FeCl 3 . 
The solution was stirred for 2-3 min and added to 100 mL of cold methanol containing 
excess tetraethylammonium chloride. The product precipitated as a yellow solid that was 
then filtered, washed with cold methanol and dried under vacuum [yield 450 mg, calc 
(obs) for calc.(obs) for CssFL^NjOFeCb: %C 61.47 (61.66) %H 6.34 (6.3 1) %N 6. 14 
(6.01)]. 

Results and Discussion 
Brewster Angle Microscopy 

Brewster angle microscopy (BAM) is an experimental technique that can give a 
macroscopic picture of an amphiphile's behavior on the water surface. The technique 
exploits the change in index of refraction that occurs at the interface of a thin film and the 
water surface by monitoring the reflected intensity of a laser source incident on the water 
surface at the critical angle (Brewster angle). Regions of a Langmuir monolayer will 
have variations in reflectivity where there are variations in film density, i.e. where there 
are variations in the index of refraction. Areas in the monolayer where the film density is 
low will reflect little light and appear black whereas areas of film with a higher density 
will appear bright. 

The BAM images taken of the Fefterpy) film taken at zero surface pressure over a 
pure water subphase and a 0.5 mM Ag(CN) 2 ~ subphase are shown in Figure 8-3. The 
image over pure water is brighter than expected for an amphiphile in an uncompressed 



145 





7T = 

pure water 



71 = 

0.5 mM 



Figure 8-3. BAM images taken for Fe(terpy) monolayers over pure water (left) and over 
a 0.5 mM KAg(CN) 2 subphase. 



state and indicates that the complex is most likely aggregated at low pressure. The 
apparent aggregation of the complex could result through n-n interactions between the 
terpyridine ligands of adjacent complexes. The image obtained for the same complex 
over a Ag(CN) 2 " subphase shows the film in a highly compressed, rigid state. The small 
jagged domains are noticeably different than the more homogeneous image seen on pure 
water and are a clear indication that the amphiphile has condensed into a solid film. 

A BAM image taken of the Ni(cyclam) complex at zero surface pressure over a 1 
mM NaNC>3 subphase is shown in Figure 8-4. The film spreads well and is very 
dynamic at low pressure, showing a liquid phase in the monolayer. The behavior of the 
monolayer is quite different when spread over a subphase containing either Ni(CN)4~ or 
Cr(CN)6 3 " ions. As shown in Figure 8-4, both films over metal cyanide solutions consist 
of highly condensed domains with rather sharp well-defined boundaries. This behavior is 
consistent with a condensation reaction between the Ni(cyclam) amphiphile and the 
aqueous Ni(CN) 4 " and Cr(CN)6 3 " ions. 



146 




1 mM Na + 



1 mM Cr(CN) 6 



1 mM Ni(CN)4 



Figure 8-4. BAM images taken for Ni(cyclam) monolayers over (left) ImM Na + , 
(center) ImM Cr(CN) 6 3 \ and (right) 1 mM Ni(CN) 4 2 " subphases. 



Infrared Spectroscopy 

The BAM images for both the Fe(terpy) and Ni(cyclam) systems indicate a 
reaction of the amphiphiles with the aqueous metal cyanide complexes. To obtain further 
evidence for cyanide bridge formation in the materials, the films were transferred to 
silicon ATR crystals for infrared spectroscopy. The FT-IR spectrum for the Fe(terpy) 
film after reaction with Ag(CNh" is compared to the FT-IR spectrum taken of KAg(CN) 2 
as a KBr pellet in Figure 8-5. The presence of the C-N stretching band at 2164 cm" 1 in 
the film is evidence that the Ag(CN)2" species has been incorporated into the film. 
Furthermore, the shift of the C-N stretching band by ~ 30 cm" 1 to higher frequency 
relative to the KAg(CN) 2 is indicative that the CN ligands have assumed a bridging 
mode. 

The FT-IR spectrum obtained for the Ni(cyclam) film transferred from a 
Ni(CN)4 2 " subphase is compared to the spectrum obtained for K2Ni(CN) 4 as a KBr pellet 
in Figure 8-6. The absorbance due to the C-N stretching mode is observed in the film and 
indicates that the Ni(CN)4 2 " species has been incorporated into the monolayer. The band 
is well resolved into two separate peaks; one occurs at the same frequency, 2124 cm" 1 , as 












< 



5 



147 



-l 1 p- 



I I 1 T 



-i r---T- ■■ t 




■ !■■■■! 



' » » 



2250 



2200 2150 



Wavenumber, cm" 



2100 



2050 



Figure 8-5. Infrared absorbance spectra of the C-N stretching region for (a) KAg(CN)2 
as a KBr pellet and (b) a monolayer film ofFe(terpy) transferred of a KAg(CN)2 
subphase. The shift to higher wavenumber in the film is indicative of cyanide bridging. 



the C-N stretching mode for the K 2 Ni(CN) 4 solid; and one that is shifted by 40 cm" 1 to 
2164 cm" . The shift to higher frequency is supportive of bridging cyanides. The near 
equal intensity of the two absorbance bands supports the target linear chain structure 
where two of the nickel cyanides are bridged and two are terminal. A similar splitting of 
the C-N stretching band has been reported in a nickel cyanide linear chain material. 248 

The FT-IR spectrum for a thin film sample of the Ni(cyclam) complex transferred 
after reaction with a Cr(CN) 6 3 " subphase is related to the FT-IR spectrum of K 3 Cr(CN) 6 
as a KBr pellet in Figure 8-7. The C-N absorbance bands at 2130 and 2160 cm' 1 confirm 
the incorporation of chromium cyanide into the film. The absorbance at 2130 cm" 1 
matches with the 2130 cm" 1 peak observed in the starting material and indicates the 



148 



0.04 



9 

c 

~v 0.02 
6 



- 

o 



0.00 



I I I 



-1 1 1— 



I I I 




■ 



2300 



2200 



2100 



2000 



Wavenumber (cm ) 



Figure 8-6. Infrared absorbance spectra of the C-N stretching region for (a) K 2 Ni(CN) 4 
as a KBr pellet and (b) a monolayer film of Ni(cyclam) transferred off a K2Ni(CN)4 
subphase. The shift to higher wavenumber in the film is indicative of cyanide bridging. 



c 

3 



- 






E 
O 



1 1 1 1 1 ■ 


-1 1 1 1 1 


/ 


\ b 




; 


J 


a 





2300 2200 2100 

Wavenumber (cm ) 



2000 



Figure 8-7. Infrared absorbance spectra of the C-N stretching region for (a) K 3 Cr(CN)6 
as a KBr pellet and (b) a monolayer film ofNi(cyclam) transferred off a K 3 Cr(CN) 6 
subphase. The shift to higher wavenumber in the film is indicative of cyanide bridging. 



149 



presence of terminal cyanides in the film. The peak at 2160 cm" 1 is shifted relative to the 
starting material and indicates the presence of cyanide bridges in the film. The presence 
of both bridged and non-bridged cyanides in the film is consistent with what would be 
expected for a honeycomb network in which three facial cyanides are terminal and the 
three opposing facial cyanides are bridges. Similar C-N stretching frequencies were 
reported for other two-dimensional honeycomb networks built on chromium hexacyanide 
and analogous nickel cyclam complexes. 211 * 213 
Grazing Incidence X-ray Diffraction 

The BAM images and FT-IR spectra for both the Fefterpy) and Ni(cyclam) 
systems are suggestive of the formation of cyanide-bridged networks at the air- water 
interface. Final confirmation of the structures being formed was sought through GIXD. 
The Fe(terpy) material was investigated in situ as a Langmuir monolayer while still on 
the water surface. The GIXD pattern obtained on the Fe(terpy) monolayer over pure 
water at the onset of surface pressure is shown in Figure 8-8. The pattern consists of a 
single broad peak at a d spacing of 4. 18 A indicating a loose organization of the alkyl 
chains. The presence of some degree of order in the film at low pressure is indicative of 
incomplete spreading of the amphiphile. This aggregation was also evident in the BAM 
image taken at low pressure in Figure 8-3. The structure of the film at the same pressure 
is clearly different when Ag(CN) 2 " ions are present in the subphase. The peak at 4. 18 A 
due to the alkyl chain packing has vanished and a series of three sharp peaks are observed 
at d spacings of 3.75 A, 3.04 A, and 2.37 A. The pattern is consistent with the formation 
of a crystalline phase at the air- water interface. Analysis of the width of the peak at 
3.04 A by the Scherrer method 147 yields a crystal coherence length of -375 A. 









150 



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300 



150 







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2.5 






£ 150 h 

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• 



| » i i i 1 1 1 1 »— j 1 f i 



h** 



'' ,l ■■■■*' ■ « i I ■ « I 



4.5 4.0 3.5 3.0 
d spacing, A 



2.5 



Figure 8-7. The GIXD pattern obtained off the water surface for the Fe(terpy) 
monolayer (bottom) over pure water at the onset of surface pressure and (top) over 1 mM 
KAg(CN)2 at the onset of surface pressure. 



Similar diffraction patterns are obtained on 30 bilayer films of the Fe(terpy) 
complex transferred at 30 mN/m over pure water and Ag(CN) 2 " subphases (Figure 8-8). 
The multilayer film transferred over pure water shows two peaks at 4.08 A and 3.50 A. 
The d spacings are typical of alkyl chain packing and the presence of two peaks in alkyl 
systems is usually indicative a nearest-neighbor tilt that leads to a centered rectangular 
cell. The size mismatch between the terpyridine head group and the alkyl chain would 
force a highly tilted aggregation of the hydrocarbon tails. The multilayer film transferred 
over Ag(CN) 2 " shows the same set of peaks as observed in the analogous Langmuir 
monolayer diffraction. A wide scan range is available for transferred films allowing for 
the detection of a fourth diffraction peak at 1 .87 A. As with the monolayer film, the 



151 



1000 r— -r 



750 
| 500 
U 250 



l l l l | — l— l — l l | I I I I | I I I 



T I I I I I I I I 



■ IliT^ 

fir"" 



A 



Ok 

3000 




_i — i — i — i — i — i — i — 1_ 



i ■ i i i i i 
4.5 4.0 3.5 3.0 



2000- 



3 10004 



o 

U 



0- 



(10) 

(0 1) 



(1-1) 
(-11) 



1 



4.5 4.0 



2.5 

— i — 



2.0 

— i — 



(11) 

(-1 -1) (2 0) 
(0 2) 



— I ■ 1 ■ 1 - 

3.5 3.0 2.5 

d spacing, A 




Figure 8-8. GIXD patterns obtained on 30 bilayer films of the Fe(terpy) complex 
transferred at 30 mN/m over (top) pure water and (bottom) a 1 mM KAg(CN)2 subphase. 
The peaks in the bottom diffraction pattern are indexed to the trigonal unit cell reported 
for AgCN as described in the text. 



absence of the diffraction peaks corresponding to the alkyl chain organization is 
indicative of an interaction between the Fe(terpy) complex and the silver cyanide ions. 
The presence of octadecanol in the multilayer film may also be responsible for the 
interruption of the alkyl chain packing; however, the presence of the alcohol did not 
affect the diffraction of the monolayer sample in a control study. Analysis of the peak 
widths for the Fe(terpy) film reacted with Ag(CN) 2 " yield an average coherence length of 
-450 A and verify the presence of well-ordered networks in the multilayer films, vide 
infra. 






2 



o 

i 2 

b 



152 



-1 1 1— 



-i 1 1 - 



-1 I L_ 



4 



6 
T(K) 



8 



-i — i — i — i- 




10 



Figure 8-9. The magnetization as a function of temperature for the Ni(cyclam) film 
reacted with Cr(CN) 6 3 ". The data are shown for two orientation of the sample with 
respect to the applied 20 G field. Open circles are with the plane of the sample surface 
perpendicular to the applied field, solid circles are for the parallel orientation. 



The Nl(cyclam) system was also subjected to GIXD studies. Diffraction scans 
were performed on both Langmuir monolayers over Ni(CN)4 " and Cr(CN)6 " subphases 
and also on 10 bilayer films prepared over Ni(CN) 4 2 " and Cr(CN) 6 3 " subphases. There 
was no evidence of diffraction in any of the systems over the 5° < 29 < 30° (14.4 A to 
2.4 A) range studied, vide infra. 

Magnetism 

The lack of structural information obtained from the GIXD experiments on the 

Ni(cyclam) - Cr(CN)6 film prompted a search for other evidence indicating the presence 
of an extended network in the film. The presence of two paramagnetic metal centers in 
the Ni(cyclam) film prepared with Cr(CN)6 3 " allows for possible magnetic exchange 



153 

interactions between neighboring metal sites. The magnetization as a function of 
temperature for the Ni(tyclam) film reacted with Cr(CN) 6 3 " is shown in Figure 8-9 in the 
form of a M*T plot. The film was 68 bilayers per side and measured 5 cm 2 . The data are 
shown for two orientations of the film with respect to the applied field. The magnetic 
response is anisotropic with a stronger magnetic response observed when the sample is 
aligned with the plane of the sample surface perpendicular to the applied field. The 
upturn in both plots below 6 K is a signature of ferromagnetic exchange interactions in 
the material. The plots show a cusp at a temperature of 2.5 K suggesting a transition to a 
long-range ordered state below T c ~ 2 K. 
Structures of the Networks 

Fe(terpy) The data presented above for the films prepared from Fe(terpy) and 
Ag(CN)2 _ provide clear evidence for the presence of an extended inorganic network in the 
films. The GIXD data do not agree with the target network described in Scheme 8-1 . 
The d-spacings for the reflections indicate a much smaller unit cell than would be 
expected for an Fe(terpy)-NC-Ag-CN-Fe(terpy)-.. . arrangement with an Fe-Fe 
separation on the order of 1 1 A. In addition, the terpyridine ligand is ~ 10 A across. 
Thus, the unit cell expected for such an arrangement would be at least 100 A 2 . The long 
coherence lengths indicated by the narrow peak widths are suggestive of inorganic 
crystallites being formed under the monolayer. The most likely structure to form under 
the monolayer would be polymeric AgCN. The crystal structure of AgCN has recently 
been reported by Bowmaker. 250 The reported structure, as shown in Figure 8-10, consists 
of isolated linear (Ag-CN-) n repeating units. The unit cell can be assigned to a trigonal 
space group with a = 3.88 A and a = 101 . 1 1°. Using these cell parameters, one can 



154 






?* 



* 



\f\ 






'■> 



:? 






& 






i> 



IN 



(S 



* N 
C 



f Ag 

N 
,'■$ & C 



k Ag 



Figure 8-10. The structure of AgCN, taken from reference 250. 



calculate the in-plane scattering expected for a series of (M) reflections. The results are 
compared to the observed reflections for the Fe(terpy)-A%CN film in Table 1 . The two 
sets of data agree very well and suggest that the diffraction observed in the film is due to 
the formation of AgCN within the film. Further evidence is provided by the FT-IR 
spectrum of the film shown in Figure 8-5. The absorbance at 2164 cm" 1 is identical to 
that reported for AgCN. 250 One thing that remains uncertain is what function the 
Fe(terpy) compound serves in the formation of the AgCN. The structure of the 
monolayer is altered in the presence of the Ag(CN)2" complex as evidenced by the 
disappearance of the alkyl chain diffraction when the film is over a silver cyanide 
subphase. The iron complex may promote the formation of the AgCN polymer by 
consumption of photo-labilized CN" ligands. The resulting cyanide-deficient silver 
complex could then polymerize with other cyanide-deficient silver complexes. The 
monolayer could also provide a high-energy surface for the subsequent crystallization of 
the AgCN. The crystallization of inorganic solids beneath Langmuir monolayers has 



155 



Table 8-1. Calculated lattice d-spacing for trigonal AgCN, a = 3.88 A a = 101. 11° (from 
data in reference 249) compared to the lattice d-spacing observed in the Fe(terpy)-AgCN 
film (Figure 8-8). 



(hk) 


Calc d-spacing (A) 


Obs d-spacing (A) 


(1 0) (0 1) 


3.81 


3.74 


(1-1) (-11) 


3.00 


3.04 


(1 1) (-1 -1) 


2.47 


2.38 


(2 0) (0 2) 


1.90 


1.85 



been reported for a number of systems and it is quite possible a similar process is 
occurring beneath the Fe(terpy) monolayer. 

Ni(cyclam). The lack of diffraction data for this system makes an unambiguous 
structural assignment impossible. However, the BAM images and infrared absorbance 
spectra for the two metal cyanide films are highly suggestive to the formation of extended 
networks in these films. The presence of both bridged and non-bridged cyanides is in 
agreement with the reported cyanide stretching band in analogous solid state materials. 
For the {Ni(cyclam)-N\(CN)4} linear chain film, the infrared data are the only available 
probes into the structure of these films, since the presence of diamagnetic square planar 
tetracyanonickelate species would discriminate against long-range magnetic order. The 
{Ni(cyclam)-Cr(CN) 6 } honeycomb networks are magnetic. The magnetic properties of 
several {Ni(cyclam)-Cr(CN) 6 } honeycomb networks are available for 
comparison. 4 "* The magnetic behavior in these materials is varied, but all show the 
presence of ferromagnetic exchange within the two-dimensional sheets. The materials 
reported by Kou and by Marvillier 213 show metamagnetic behavior due to the presence 



156 

of inter-planar antiferromagnetic coupling below Tn ~ 12 K. The material described by 
Ferlay 42 showed the presence of ferromagnetic coupling, but no transition to an ordered 
state above 2 K. The film we have prepared is not isostructural in the strictest sense with 
the analogous solids since the film consists of bilayers effectively isolated by the 
hydrocarbon inter-bilayer region. This structural difference effectively lowers the 
dimensionality of the material and may disrupt any long-range antiferromagnetic 
interactions. As isolated two-dimensional networks, the ordering temperature is expected 
to be low. Further magnetic measurements near the apparent transition at 2 K with AC 
magnetometry would help discern if the cusp in the M*T data is due to long range 
magnetic ordering. The presence of such order would be clear evidence of a structurally 
coherent network; however, the structure of the network would still be open to question 
without the necessary diffraction data. The absence of diffraction in these materials is 
not certain since the rather large unit cells would place the primary diffraction peaks at 
low angles in the GIXD scan. Unfortunately, the low angle region has a very high 
background due to scattering in the incident beam. Repeating the diffraction experiments 
at as low energy as possible would aid in moving the larger d-space peaks to higher 
angles and thereby reduce the background contribution. In closing, it is also worth noting 
that the only structures reported to date for the product of a reaction between M(cyclam) 
complexes and chromium(IH)hexacyanide have been the two-dimensional honeycomb 
networks, suggesting that this is the preferred product of the reaction. 

Conclusions 

The reaction of two low symmetry amphiphilic transition metal complexes with 
aqueous metal cyanides has been performed at the air water interface. One system 
involved the reaction of a Langmuir monolayer of an amphiphilic "T-shaped" 



157 

Fe(III)(mono-terpyridine)trichloride complex with aqueous linear dicyanoargentate ions. 
Characterization of the resulting film by FT-IR and GIXD showed that product of the 
reaction was not the expected Fe(terpy) -NC-Ag linear chain polymer, but instead 
polymeric AgCN crystallites which formed under the surface of the monolayer. The 
other system studied involved the reaction of an amphophilic nickel(cyclam) complex 
contained as a Langmuir monolayer aqueous square planar tetracyanonickelate and 
octahedral chromium(III)hexacyanide ions. Characterization of the resulting 
Ni(cyclam) Ni(CN)4 film by BAM and FT-IR gave evidence for the formation of a 
cyanide-bridged linear chain polymer; however, direct structural confirmation with the 
material was hampered by the lack of significant diffraction from GIXD studies. The 
lack of sufficient x-ray diffraction for the Ni(cyclam)Cr{CW)e film prevented a detailed 
structure assignment for the material; however the FT-IR spectrum and magnetic 
behaviors observed were consistent with the target two-dimensional honeycomb network 
structure. 



LIST OF REFERENCES 



(1) Lehn, J. M; Atwood, J. L ; Davies, J. E. D.; McNicol, D. D ; Vogtle, V., Eds. 
Comprehensive Supramoleciilar Chemistry, Pergamon Press: Oxford, U. K., 1996. 

(2) Philp, D.; Stoddart, J. F. Angew. Chem. Int. Edit. 1996, 35, 1 155-1 196. 

(3) Greig, L. M; Philp, D. Chem. Soc. Rev. 2001, 30, 287-302. 

(4) Fredericks, J. R.; Hamilton, A. D. In Supermolecular Control ofStucture and 
Reactivity, Hamilton, A. D., Ed.; John Wiley and Sons: New York, 1996, p 1. 

(5) Baxter, P. N. W. In Comprehensive Supramolecular Chemistry, Sauvage, J. P., 
Hosseini, M. W., Eds.; Permagon Press: Oxford, U. K., 1996; Vol. 9, p 165. 

(6) Fujita, M. In Comprehensive Supramolecular Chemistry, Sauvage, J. P., Hosseini, 
M. W., Eds.; Permagon Press: Oxford, U. K., 1996; Vol. 9, p 253. 

(7) Funeriu, D. P.; Lehn, J. M ; Baum, G ; Fenske, D. Chem. Eur. J. 1997, 3, 99-104. 

(8) Vance, A. L.; Alcock, N. W.; Busch, D. H; Heppert, J. A. Inorg. Chem. 1997, 36, 
5132-5134. 

(9) Yaghi, O. M ; Li, H. L.; Groy, T. L. Inorg. Chem. 1997, 36, 4292-4293. 

(10) Yaghi, O. M.; Li, H. L.; Davis, C; Richardson, D.; Groy, T. L. Ace. Chem. Res. 
1998, 31, 474-484. 

(11) Lopez, S.; Kahraman, M.; Harmata, M.; Keller, S. W. Inorg. Chem. 1997, 36, 
6138-6140. 

(12) Lu, J.; Paliwala, T ; Lim, S. C; Yu, C; Niu, T. Y.; Jacobson, A. J. Inorg. Chem. 
1997, 36, 923-929. 

(13) Dunbar, K. R.; Heintz, R. A. Prog. Inorg. Chem. 1997, 45, 283-391 . 

(14) Stang, P. J.; Olenyuk, B. Ace. Chem. Res. 1997, 30, 502-5 18. 

(15) Stang, P. J. Chem. Eur. J. 1998, 4, 19-27. 

(16) Clemente-Juan, J. M.; Coronado, E. Coord. Chem. Rev. 1999, 195, 361-394. 

(17) Sessoli, R ; Tsai, H. L.; Schake, A. R.; Wang, S. Y.; Vincent, J. B.; Folting, K.; 
Gatteschi, D ; Christou, G; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 
1804-1816. 

(18) Shores, M. P.; Sokol, J. J.; Long, J. R. J. Am. Chem. Soc. 2002, 124, 2279-2292. 

(19) Berseth, P. A; Sokol, J. J.; Shores, M. P.; Heinrich, J. L.; Long, J. R. J. Am. Chem. 
Soc. 2000, 122, 9655-9662. 

(20) Heinrich, J. L ; Sokol, J. J.; Hee, A G; Long, J. R. J. Solid State Chem. 2001, 159, 
293-301. 

(21) Larionova, J.; Gross, M; Pilkington, M.; Andres, H; Stoeckli-Evans, H; Gudel, 
H. U.; Decurtins, S. Angew. Chem.-Int. Edit. 2000, 39, 1605-1609. 

(22) Rogez, G; Parsons, S.; Paulsen, C; Villar, V; Mallah, T. Inorg. Chem. 2001, 40, 
3836-3837. 

(23) Parker, R. J.; Spiccia, L.; Berry, K. J.; Fallon, G D.; Moubaraki, B.; Murray, K. S. 
Chem. Commun. 2001, 333-334. 



158 



159 



(24) Scuiller, A.; Mallah, T.; Verdaguer, M.; Nivorozkhin, A.; Tholence, J. L.; Veillet, 
P. New J. Chem. 1996, 20, 1-3. 

(25) Mallah, T.; Auberger, C; Verdaguer, M; Veillet, P. Chem. Commun. 1995, 61-62. 

(26) Oshio, H.; Tamada, O.; Onodera, H.; Ito, T.; Ikoma, T.; Tero-Kubota, S. Inorg. 
Chem. 1999, 38, 5686-5689. 

(27) Oshio, H.; Onodera, H.; Tamada, O; Mizutani, H.; Hikichi, T.; Ito, T. Chem.Eur. 
J. 2000, 6, 2523-2530. 

(28) Fujita, M; Ogura, K. Coord. Chem. Rev. 1996, 148, 249-264. 

(29) Fujita, M; Yu, S. Y.; Kusukawa, T.; Funaki, H.; Ogura, K.; Yamaguchi, K. 
Angew. Chem. Int. 1998, 37, 2082-2085. 

(30) Kou, H. Z.; Liao, D. Z.; Jiang, Z. H ; Yan, S. P.; Wu, Q. J.; Gao, S.; Wang, G. L. 
Inorg. Chem. Commun. 2000, 3, 151-154. 

(31) Ohba, M; Usuki, N.; Fukita, N.; Okawa, H. Inorg. Chem. 1998, 37, 3349-3354. 

(32) Cernak, J.; Orendac, M; Potocnak, I.; Chomic, J.; Orendacova, A.; Skorsepa, J.; 
Feher, A. Coord Chem. Rev. 2002, 224, 51-66. 

(33) Ohba, M; Fukita, N.; Okawa, H. J. Chem. Soc. Dalton Trans. 1997, 1733-1737. 

(34) Ohba, M; Maruono, N.; Okawa, H.; Enoki, T.; Latour, J. M. J. Am. Chem. Soc. 
1994,116, 11566-11567. 

(35) Zhan, S. Z ; Guo, D.; Zhang, X. Y.; Du, C. X.; Zhu, Y ; Yang, R. N. Inorg. Chim. 
Acta 2000, 298, 57-62. 

(36) Clearfield, A. In Prog. Inorg. Chem. 1998, 47, 371-510. 

(37) Ohba, M.; Okawa, H.; Ito, T.; Ohto, A. Chem. Commun. 1995, 1545-1546. 

(38) Ohba, M; Okawa, H.; Fukita, N.; Hashimoto, Y. J. Am. Chem. Soc. 1997, 119, 
1011-1019. 

(39) Kou, H. Z.; Tang, J. K; Liao, D. Z ; Gao, S.; Cheng, P.; Jiang, Z. H.; Yan, S. P.; 
Wang, G. L.; Chansou, B.; Tuchagues, J. P. Inorg. Chem. 2001, 40, 4839-4844. 

(40) Kou, H. Z ; Bu, W. M.; Liao, D. Z ; Jiang, Z. H.; Yan, S. P.; Fan, Y. G; Wang, G. 
L. J. Chem. Soc. Dalton Trans. 1998, 4161-4164. 

(41) Colacio, E.; Dominguez-Vera, J. M.; Ghazi, M.; Kivekas, R.; Lloret, F.; Moreno, 
J. M.; Stoeckli-Evans, H. Chem. Commun. 1999, 987-988. 

(42) Ferlay, S.; Mallah, T.; Vaissermann, J.; Bartolome, F.; Veillet, P.; Verdaguer, M. 
Chem. Commun. 1996, 2481-2482. 

(43) Kou, H. Z.; Gao, S.; Bu, W. M.; Liao, D. Z.; Ma, B. Q.; Jiang, Z. H.; Yan, S. P.; 
Fan, Y. G; Wang, G. L. J. Chem. Soc. Dalton Trans. 1999, 2477-2480. 

(44) Laget, V; Homick, C; Rabu, P.; Drillon, M.; Ziessel, R. Coord. Chem. Rev. 1998, 
180, 1533-1553. 

(45) Batten, S. R; Robson, R. Angew. Chem. Int. Edit. 1998, 37, 1461-1494. 

(46) Blake, A. J.; Champness, N. R.; Hubberstey, P.; Li, W. S.; Withersby, M. A.; 
Schroder, M. Coord. Chem. Rev. 1999, 183, 117-138. 

(47) Swiegers, G. F.; Malefetse, T. J. Chem. Rev. 2000, 100, 3483-3537. 

(48) Xia, Y. N.; Whitesides, G. M. Angew. Chem. Int. Edit. 1998, 37, 551-575. 

(49) Kahn, O. Molecular Magnetism, VCH: New York, 1993. 

(50) Coronado, E.; Galan-Mascaros, J. R.; Gomez-Garcia, C. J.; Laukhin, V. Nature 
2000, 408, 447-449. 

(51) Ferlay, S.; Mallah, T.; Ouahes, R.; Veillet, P.; Verdaguer, M. Nature 1995, 378, 
701-703. 



160 



(52) Mallah, T.; Thiebaut, S.; Verdaguer, M.; Veillet, P. Science 1993, 262, 1554-1557. 

(53) Entley, W. R ; Girolami, G. S. Science 1995, 268, 397-400. 

(54) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 27 J, 49-5 1 . 

(55) Sato, O.; Iyoda, T ; Fujishima, A.; Hashimoto, K. Science 1996, 272, 704-705. 

(56) Mizuno, M; Ohkoshi, S.; Hashimoto, K. Adv. Mater. 2000, 12, 1955-+. 

(57) Shipway, A. N; Willner, I. Ace. Chem. Res. 2001, 34, 421-432. 

(58) Armand, F.; Albouy, P. A.; Da Cruz, F.; Normand, M.; Hue, V.; Goron, E. 
Langmuir 2001, 17, 3431-3437. 

(59) Bowden, N; Terfort, A; Carbeck, J.; Whitesides, G. M. Science 1997, 276, 233- 
235. 

(60) Bowden, N; Choi, I. S.; Grzybowski, B. A; Whitesides, G M. J. Am. Chem. Soc. 
1999,727,5373-5391. 

(61) Bowden, N.; Arias, F.; Deng, T.; Whitesides, G M. Langmuir 2001, 77, 1757- 
1765. 

(62) Weissbuch, I.; Baxter, P. N. W.; Cohen, S.; Cohen, H; Kjaer, K.; Howes, P. B.; 
Als-Nielsen, J.; Hanan, G. S.; Schubert, U. S.; Lehn, J. M.; Leiserowitz, L.; Lahav, 
M. J. Am. Chem. Soc. 1998, 120, 4850-4860. 

(63) Weissbuch, I.; Baxter, P. N. W.; Kuzmenko, I.; Cohen, H; Cohen, S.; Kjaer, K.; 
Howes, P. B.; Als-Nielsen, J.; Lehn, J. M.; Leiserowitz, L.; Lahav, M. Chem. Eur. 
J. 2000, 6, 725-734. 

(64) Mingotaud, C; Lafuente, C; Amiell, J.; Delhaes, P. Langmuir 1999, 15, 289-292. 

(65) Huo, Q.; Russell, K. C; Leblanc, R. M. Langmuir 1998, 14, 2174-2186. 

(66) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett 
to Self-Assembly; Academic Press: Boston, 1991. 

(67) Roberts, C. G Langmiur-Blodgett Films, Plenum Press: New York, 1990. 

(68) Langmuir, I. J. Am. Chem. Soc. 1917, 39, 1848-1906. 

(69) Jones, L. H. Inorg. Chem. 1963, 2, 111-&.. 

(70) Bignozzi, C. A; Argazzi, R.; Schoonover, J. R; Gordon, K. C; Dyer, R.B.; 
Scandola, F. Inorg. Chem. 1992, 31, 5260-5267. 

(71) Alvarez, S.; Lopez, C; Bermejo, M. J. Transit. Met. Chem. 1984, 9, 123-126. 

(72) Shriver, D. F. J. Am. Chem. Soc. 1963, 85, 1405-&. 

(73) Dows, D. A.; Haim, A.; Wilmarth, W. K. J. Inorg. Nucl. Chem. 1961, 27, 33-37. 

(74) Duke, P. J. Synchrotron Radiation: Production and Properties; Oxford University 
Press: New York, 2000. 

(75) Advanced Photon Source, http://aps.anl.gov . accessed September 13, 2002. 

(76) Materials Research Collaborative Access Team, http : //ixs. csrri . iit . edu/mrcat/ . 
accessed September 13, 2002. 

(77) Advanced Photon Source, http ://www. imca. aps. anl . gov/mx/. accessed September 
13, 2002. 

(78) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K; Leveiller, F.; Lahav, M.; Leiserowitz, 
L. Phys. Rep. 1994, 246, 252-313. 

(79) Kaganer, V. M.; Mohwald, H; Dutta, P. Rev. Mod. Phys. 1999, 71, 779-819. 

(80) Kuzmenko, I.; Rapaport, H.; Kjaer, K; Als-Nielsen, J.; Weissbuch, I.; Lahav, M.; 
Leiserowitz, L. Chem. Rev. 2001, 707, 1659-1696. 

(81) International XAFS Society, http://ixs.iit.edu/ . accessed September 13, 2002. 



161 



(82) Teo, B. K. EXAFS: Basic Principles and Data Analysis, Springer- Verlag: New 
York, 1986. 



(83 
(84 
(85 

(86 
(87 
(88 
(89 

(90 
(91 

(92 
(93 
(94 



(95 

(96 

(97 
(98 
(99 

(100 

(101 

(102 
(103 
(104 
(105 

(106 

(107 
(108 

(109 

(110 
(111 
(112 



Sayers, D. E.; Stern, E. A.; Lytle, F. W. Phys. Rev. Lett. 1971, 27, 1204-&. 

Ressler, T. J. Synchrot. Radiat. 1998, 5, 118-122. 

Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C ; Eller, M. J. Phys. Rev. B 

1995, 52, 2995-3009. 

Ankudinov, A.; "Ph. D. Thesis"; University of Washington, 1996. 

Clearfie.A; Smith, G. D. Inorg. Chem. 1969, 8, 431-&. 

Clearfield, A. Chem. Rev. 1988, 88, 125-148. 

Alberti, G; Costantino, U.; Allulli, S.; Tomassini, N. J. Inorg. Nucl. Chem. 1978, 

40, 1113-1117. 

Cao, G.; Lee, H.; Lynch, V. M; Mallouk, T. E. Solid State Ion. 1988, 26, 63-69. 

Cao, G; Lee, H ; Lynch, V. M; Mallouk, T. E. Inorg. Chem. 1988, 27, 2781- 

2785. 

Cunningham, D.; Hennelly, P. J. D. Inorg. Chim. Acta 1979, 37, 95-102. 

Ortizavila, Y ; Rudolf, P. R; Clearfield, A. Inorg. Chem. 1989, 28, 2137-2141. 

Caldwell, W. B ; Campbell, D. J.; Chen, K. M ; Herr, B. R; Mirkin, C. A; Malik, 

A; Durbin, M. K; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 777, 6071- 

6082. 

Katz, H. E.; Scheller, G; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; 

Chidsey, C. E. D. Science 1991, 254, 1485-1487. 

Katz, H. E.; Schilling, M. L.; Chidsey, C. E. D.; Putvinski, T. M.; Hutton, R. S. 

Chem. Mat. 1991, 3, 699-703. 

Katz, H. E.; Wilson, W. L.; Scheller, G. J. Am. Chem. Soc. 1994, 776, 6636-6640. 

Dines, M. B ; Digiacomo, P. M. Inorg. Chem. 1981, 20, 92-97. 

Poojary, D. M.; Vermeulen, L. A.; Vicenzi, E.; Clearfield, A.; Thompson, M. E. 

Chem. Mat. 1994, 6, 1845-1849. 

Visser, D.; Carling, S. G; Day, P.; Deportes, J. J. Appl. Phys. 1991, 69, 6016- 

6018. 

Carling, S. G; Day, P.; Visser, D ; Kremer, R. K. J. Solid State Chem. 1993, 706, 

111-119. 

Carling, S. G; Day, P.; Visser, D. Inorg. Chem. 1995, 34, 3917-3927. 

Carling, S. G; Day, P.; Visser, D. J. Phys. Condes. Matter 1995, 7, L109-L1 13. 

Rabu, P.; Janvier, P.; Bujoli, B. J. Mater. Chem. 1999, 9, 1323-1326. 

Lebideau, J.; Payen, C; Bujoli, B ; Palvadeau, P.; Rouxel, J. J. Magn. Magn. 

Mater. 1995, 140, 1719-1720. 

Bujoli, B.; Pena, O.; Palvadeau, P.; Lebideau, J.; Payen, C; Rouxel, J. Chem. Mat. 

1993, 5, 583-587. 

Gerbier, P.; Guerin, C; Le Bideau, J.; Valle, K. Chem. Mat. 2000, 72, 264-267. 

Fanucci, G E.; Krzystek, J.; Meisel, M. W.; Brunei, L. C; Talham, D. R. J. Am. 

Chem. Soc. 1998, 720, 5469-5479. 

Fanucci, G E.; Petruska, M. A.; Meisel, M. W.; Talham, D. R. J. Solid State 

Chem. 1999, 145, 443-451. 

Byrd, H.; Pike, J. K.; Talham, D. R. Chem. Mat. 1993, 5, 709-715. 

Byrd, H.; Pike, J. K; Talham, D. R. J. Am. Chem. Soc. 1994, 776, 7903-7904. 

Byrd, H.; Pike, J. K.; Talham, D. R Synth. Met. 1995, 77, 1977-1980. 



162 



(113) Fanucci, G. E.; Seip, C. T.; Petruska, M. A.; Nixon, C. M; Ravaine, S.; Talham, 
D. R. Thin Solid Films 1998, 329, 331-335. 

(1 14) Fanucci, G. E.; Talham, D. R. Langmuir 1999, 15, 3289-3295. 

(115) Petruska, M. A.; Fanucci, G. E.; Talham, D. R. Chem. Mat. 1998, 10, 177-189. 

(1 16) Petruska, M. A.; Fanucci, G E.; Talham, D. R. Thin Solid Films 1998, 329, 131- 
135. 

(1 17) Petruska, M. A.; Talham, D. R Chem. Mat. 1998, 10, 3672-3682. 

(118) Petruska, M. A. ; Talham, D. R. Langmuir 2000, 16, 5 1 23-5 1 29. 

(1 19) Petruska, M. A; Watson, B. C; Meisel, M. W.; Talham, D. R. Chem. Mat. 2002, 
7^,2011-2019. 

(120) de Jongh, L. J. In Magnetic Properties of Layered Transition Metal Compounds; 
de Jongh, L. J., Ed.; Kluwer Academic Publishers: Dordrecht, 1990, pp 1-51. 

(121) Rubenacker, G V.; Raffaelle, D. P.; Drumheller, J. E ; Emerson, K. Phys. Rev. B 
1988, 37, 3563-3568. 

(122) Zenmyo, K.; Kubo, H. J. Phys. Soc. Jpn. 1995, 64, 1320-1325. 

(123) DeFotis, G. C; Just, E. ML; Pugh, V. J.; Coffey, G A; Hogg, B. D.; Fitzhenry, S. 
L.; Marmorino, J. L.; Krovich, D. J.; Chamberlain, R. V. J. Magn. Magn. Mater. 
1999, 202, 27-46. 

(124) DeFotis, G. C; Coker, G. S.; Jones, J. W.; Branch, C. S.; King, H. A.; Bergman, J. 
S.; Lee, S.; Goodey, J. R Phys. Rev. B 1998, 58, 12178-12192. 

(125) Defotis, G. C; Mantus, D. S.; McGhee, E. ML; Echols, K. R; Wiese, R. S. Phys. 
Rev. B 1988, 38, 1 1486-1 1499. 

(126) Mydosh, J. A. Spin Glasses, Taylor and Francis: Washington, DC, 1993. 

(127) Nguyen, H. C; Goodenough, J. B. Phys. Rev. B 1995, 52, 8776-8787. 

(128) Nuttall, C. J.; Day, P. Chem. Mat. 1998, 10, 3050-3057. 

(129) Fanucci, G. E.; Culp, J. T.; Watson, B. C; Backov, R; Ohnuki, H; Talham, D. R; 
Meisel, M. W. Physica B 2000, 284, 1499-1500. 

(130) Signore, P. J. C; "Ph. D. Thesis"; University of Florida, 1994. 

(131) Ward, B. H; Granroth, G E.; Walden, J. B.; Abboud, K. A; Meisel, M. W.; 
Rasmussen, P. G; Talham, D. R. J. Mater. Chem. 1998, 8, 1373-1378. 

(132) Seip, C. T.; Byrd, H; Talham, D. R. Inorg Chem. 1996, 35, 3479-3483. 

(133) Lines, M. E. J. Phys. Chem. Solids 1970, 31, 101. 

(134) Sykes, M. F.; Fisher, M. E. Physica 1962, 28, 919. 

(135) Navarro, R. In Magnetic Properties of Layered Transition Metal Compounds, de 
Jongh, L. J., Ed.; Kluwer Academic Publishers: Dordrecht, 1990. 

(136) Fishman, S.; Aharony, A. Phys. Rev. B 1978, 18, 3507-3520. 

(137) Katsumata, K; Kobayashi, ML; Sato, T.; Miyako, Y. Phys. Rev. B 1979, 19, 2700- 
2703. 

(138) Katsumata, K; Shapiro, S. M.; Matsuda, ML; Shirane, G.; Tuchendler, J. Phys. 
Rev. B 1992, 46, 14906-14908. 

(139) Wong, P.; Horn, P. ML; Birgeneau, R. J.; Shirane, G Phys. Rev. B 1983, 27, 428- 
447. 

(140) Coronado, E.; Galan-Mascaros, J. R ; Gomez-Garcia, C. J.; Martinez- Agudo, J. M. 
Adv. Mater. 1999, 11, 558-561. 

(141) Defotis, G C; Pohl, C; Pugh, S. A.; Sinn, E. J. Chem. Phys. 1984, 80, 2079-2086. 



163 



(142) Richardson, R. C; Smith, E. N. Experimental Techniques in Condensed Matter 
Physics at Low Temperatures, Addison- Wesley Pub. Co.: Redwood City, CA, 
1988. 

(143) Nguyen, H. C; Goodenough, J. B. Phys. Rev. B 1995, 52, 324-334. 

(144) Byrd, H; Whipps, S.; Pike, J. K.; Ma, J. F.; Nagler, S. E.; Talham, D. R. J. Am. 
Chem. Soc. 1994, 116, 295-301. 

(145) Seip, C. T.; Granroth, G. E.; Meisel, M. W.; Talham, D. R. J. Am. Chem. Soc. 
1997, 119, 7084-7094. 

(146) Carino, S. R.; Tostmann, H ; Underhill, R. S.; Logan, J.; Weerasekera, G; Culp, 
J.; Davidson, M; Duran, R. S. J. Am. Chem. Soc. 2001, 123, 767-768. 

(147) Guinier, A. X-ray Diffraction, Freeman: San Francisco, 1968 

(148) Hammond, D. B.; Rayment, T.; Dunne, D ; Hodge, P.; Ali-Adib, Z.; Dent, A. 
Langmuir 1998, 14, 5896-5899. 

(149) Cao, G; Lynch, V. M ; Swinnea, J. S.; Mallouk, T. E. Inorg. Chem. 1990, 29, 
2112-2117. 

(150) Wang, R. C; Zhang, Y. P.; Hu, H. L.; Frausto, R. R; Clearfield, A. Chem. Mat. 
1992, 4, 864-871. 

(151) Ozin, G. A. Adv. Mater. 1992, 4, 612-649. 

(152) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208-21 1. 

(153) Harrison, R. M.; Brotin, T.; Noll, B. C; Michl, J. Organometallics 1997, 16, 3401- 
3412. 

(154) Magnera, T. F.; Pecka, J.; Vacek, J.; Michl, J. In Nanostructured Materials, 
American Chemical Society: Washington, DC, 1997, p 231. 

(155) Magnera, T. F.; Peslherbe, L. M.; Korblova, E.; Michl, J. J. Organomet. Chem. 
1997, 548, 83-89. 

(156) Pospisil, L.; Heyrovsky, M.; Pecka, J.; Michl, J. Langmuir 1997, 13, 6294-6301. 

(157) Jacquemain, D.; Wolf, S. G; Leveiller, F.; Deutsch, M.; Kjaer, K; Alsnielsen, J.; 
Lahav, M.; Leiserowitz, L. Angew. Chem. Int. Edit. 1992, 31, 130-152. 

(158) Leveiller, F.; Bohm, C; Jacquemain, D ; Mohwald, H; Leiserowitz, L.; Kjaer, K.; 
Alsnielsen, J. Langmuir 1994, 10, 819-829. 

(159) Weissbuch, I.; Guo, S.; Edgar, R.; Cohen, S.; Howes, P.; Kjaer, K; Als-Nielsen, 
J.; Lahav, M.; Leiserowitz, L. Adv. Mater. 1998, 10, 117-121. 

(160) Hensel, V.; Godt, A; Popovitz-Biro, R; Cohen, H; Jensen, T. R; Kjaer, K; 
Weissbuch, I.; Lifshitz, E ; Lahav, M. Chem. Eur. J. 2002, 8, 1413-1423. 

(161) Landau, E. M.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. Nature 1985, 
318, 353-356. 

(162) Frostman, L. M.; Ward, M. D. Langmuir 1997, 13, 330-337. 

(163) Whipps, S.; Khan, S. R.; O'Palko, F. J.; Backov, R.; Talham, D. R. J. Cryst. 
Growth 1998, 192, 243-249. 

(164) Hanan, G. S.; Volkmer, D.; Schubert, U. S.; Lehn, J. M.; Baum, G; Fenske, D. 
Angew. Chem. Int. Edit. 1997, 36, 1842-1844. 

(165) Baxter, P. N. W.; Lehn, J. M.; Fischer, J.; Youinou, M. T. Angew. Chem. Int. Edit. 
1994, 33, 2284-2287. 

(166) Baxter, P. N. W.; Lehn, J. M.; Kneisel, B. O.; Fenske, D. Angew. Chem. Int. Edit. 
1997,36, 1978-1981. 

(167) Michl, J.; Magnera, T. F. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 4788-4792. 



164 



(168) Varaksa, N.; Pospisil, L.; Magnera, T. F.; Michl, J. Proc. Natl. Acad. Sci. U. S. A. 
2002, 99, 5012-5017. 

(169) Gadet, V.; Mallah, T.; Castro, L; Verdaguer, M; Veillet, P. J. Am. Chem. Soc. 
1992, 77-/, 9213-9214. 

(170) Re, N.; Gallo, E.; Floriani, C; Miyasaka, H ; Matsumoto, N. Inorg. Chem. 1996, 
35, 6004-6008. 

(171) Miyasaka, H; Matsumoto, N.; Okawa, H.; Re, N.; Gallo, E.; Floriani, C. J. Am. 
Chem. Soc. 1996, 118, 981-994. 

(172) Zhong, Z. J.; Seino, H.; Mizobe, Y.; Hidai, M.; Fujishima, A.; Ohkoshi, S.; 
Hashimoto, K. J. Am. Chem. Soc. 2000, 122, 2952-2953. 

(173) Hatlevik, O ; Buschmann, W. E.; Zhang, J.; Manson, J. L.; Miller, J. S. Adv. 
Mater. 1999,77,914-918. 

(174) Holmes, S. M.; Girolami, G S. J. Am. Chem. Soc. 1999, 727, 5593-5594. 

(175) Posiomek, E. J. J. Org. Chem. 1963, 28, 590-591. 

(176) Ruiz, A.; Rocca, P.; Marsais, F.; Godard, A.; Queguiner, G. Tetrahedron Lett. 
1997, 38, 6205-6208. 

(177) Armand, F.; Sakuragi, H; Tokumaru, K. New J. Chem. 1993, 77, 351-356. 

(178) Kern, W. J. Electrochem. Soc. 1990, 137, 1887-1892. 

(179) Maoz, R; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465-496. 

(180) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674-676. 

(181) Hrepic, N. V. ; Malin, J. M. Inorg. Chem. 1 979, 18, 409-4 1 3 . 

(182) Armand, F.; Sakuragi, H; Tokumaru, K. J. Chem. Soc. Faraday Trans. 1993, 89, 
1021-1024. 

(183) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 7, 1. 

(184) Laibinis, P. E.; Bain, C. D ; Whitesides, G M. J. Phys. Chem. 1991, 95, 7017- 
7021. 

(185) Brundle, C. R; Hopster, H; Swalen, J. D. J. Chem. Phys. 1979, 70, 5190-5196. 

(186) Giorgetti, M.; Berrettoni, M.; Filipponi, A.; Kulesza, P. J.; Marassi, R. Chem. 
Phys. Lett. 1997, 275, 108-112. 

(187) Zhang, H. H; Filipponi, A; DiCicco, A; Scott, M. J.; Holm, R. H; Hedman, B.; 
Hodgson, K. O. J. Am. Chem. Soc. 1997, 779, 2470-2478. 

(188) Yokoyama, T.; Ohta, T.; Sato, O.; Hashimoto, K. Phys. Rev. B 1998, 58, 8257- 
8266. 

(189) Armand, F.; Okada, S.; Yase, K.; Matsuda, H; Nakanishi, H; Sakuragi, H.; 
Tokumaru, K. Jpn. J. Appl. Phys. 1993, 32, 1 186-1 190. 

(190) Mingotaud, A. F.; Mingotaud, C; Patterson, L. K. Handbook of Monolayers 1st 
ed.; Academic Press: London, 1993; Vol. 1. 

(191) Ludi, A. ; Gudel, H. U. Struct. Bonding 1 973, 1-1,1. 

(192) Gadet, V.; Bujoli-Doeuff, M.; Force, L.; Verdaguer, M.; Makhi, E.; Deroy, A.; 
Besse, J. P.; Chappert, C; Veillet, P.; Renard, J. P.; Beauvillian, P. In Molecular 
Magnetic Materials, Series E ed.; Gatteschi, D., Kahn, O., Miller, J. S., Palacio, 
F., Eds.; Kluwer: Dordrecht, 1991; Vol. 198, p 281. 

(193) Juszczyk, S.; Johansson, C; Hanson, M.; Ratuszna, A.; Malecki, G J. Phys.- 
Condes. Matter 1994, 6, 5697-5706. 

( 1 94) Fukita, N. ; Ohba, M. ; Okawa, H. Mol. Cryst. Liquid Cryst. 2000, 342, 2 1 7-224. 



165 



(195) Verdaguer, M; Bleuzen, A.; Marvaud, V.; Vaissermann, J.; Seuleiman, M; 
Desplanches, C; Scuiller, A.; Train, C; Garde, R ; Gelly, G.; Lomenech, C; 
Rosenman, I.; Veillet, P.; Cartier, C; Villain, F. Coord. Chem. Rev. 1999, 192, 
1023-1047. 

(196) Ohba, M ; Okawa, H. Coord. Chem. Rev. 2000, 198, 313-328. 

(197) Kahn, O.; Larionova, J.; Ouahab, L. Chem. Commun. 1999, 945-952. 

(198) Koga, A.; Kawakami, N. J. Phys. Soc. Jpn. 2000, 69, 1834-1836. 

( 1 99) Takushima, Y. ; Koga, A. ; Kawakami, N. Phys. Rev. B 2000, 61, 1 5 1 89- 1 5 1 95 . 

(200) Lehn, J. M. Angew. Chem. Int. Edit. 1990, 29, 1304-1319. 

(201) Olenyuk, B.; Whiteford, J. A.; Fechtenkotter, A; Stang, P. J. Nature 1999, 398, 
796-799. 

(202) Dinolfo, P. H; Hupp, J. T. Chem. Mat. 2001, 13, 3 1 13-3 125. 

(203) Ziener, U.; Lehn, J. M.; Mourran, A; Moller, M. Chem. Eur. J. 2002, 8, 951-957. 

(204) Semenov, A.; Spatz, J. P.; Moller, M.; Lehn, J. M; Sell, B.; Schubert, D.; Weidl, 
C. H.; Schubert, U. S. Angew. Chem. Int. Edit. 1999, 38, 2547-2550. 

(205) Culp, J. T.; Park, J. H; Stratakis, D ; Meisel, M. W.; Talham, D. R. J. Am. Chem. 
Soc. 2002, 124, 10083-10090. 

(206) Bertran, J. F.; Pascual, J. B.; Hernandez, M.; Rodriguez, R. Reactivity of Solids 
1988, 5, 95-100. 

(207) Sato, O.; Einaga, Y.; Fujishima, A; Hashimoto, K. Inorg. Chem. 1999, 38, 4405- 
4412. 

(208) Carlin, R. L. Magnetochemistry, Springer: Berlin, 1986. 

(209) Buser, H. J.; Schwarzenbach, D ; Petter, W.; Ludi, A. Inorg. Chem. 1977, 16, 
2704-2710. 

(210) Kou, H. Z ; Bu, W. M; Gao, S.; Liao, D. Z.; Jiang, Z. H; Yan, S. P.; Fan, Y. G; 
Wang, G L. J. Chem. Soc. Dalton Trans. 2000, 2996-3000. 

(21 1) Kou, H. Z.; Gao, S.; Bai, O.; Wang, Z. M. Inorg. Chem. 2001, 40, 6287-6294. 

(212) Kou, H. Z.; Gao, S.; Ma, B. Q ; Liao, D. Z. Chem. Commun. 2000, 1309-13 10. 

(213) Marvilliers, A.; Parsons, S.; Riviere, E.; Audiere, J. P.; Kurmoo, M.; Mallah, T. 
Eur. J. Inorg. Chem. 2001, 1287-1293. 

(214) Re, N.; Crescenzi, R.; Floriani, C; Miyasaka, H; Matsumoto, N. Inorg. Chem. 
1998, 37, 2717-2722. 

(215) Dujardin, E.; Ferlay, S.; Phan, X.; Desplanches, C; Moulin, C. C. D.; Sainctavit, 
P.; Baudelet, F.; Dartyge, E.; Veillet, P.; Verdaguer, M. J. Am. Chem. Soc. 1998, 
120, 11347-11352. 

(216) Buschmann, W. E.; Ensling, J.; Gutlich, P.; Miller, J. S. Chem. Eur. J. 1999, 5, 
3019-3028. 

(2 1 7) Buschmann, W. E. ; Miller, J. S . Inorg. Chem. 2000, 39, 24 1 1 -242 1 . 

(218) Mathieu, R.; Jonsson, P.; Nam, D. N. H; Nordblad, P. Phys. Rev. B 2001, 6309, 
art. no. -092401. 

(219) Dormann, J. L.; Fiorani, D.; Tronc, E. Adv. Chem. Phys. 1997, 98, 283-494. 

(220) Mortimer, R. J. Chem. Soc. Rev. 1997, 26, 147-156. 

(221) Itaya, K.; Uchida, I.; Neff, V. D. Ace. Chem. Res. 1986, 19, 162-168. 

(222) Carpenter, M. K; Conell, R. S. J. Electrochem. Soc. 1990, 137, 2464-2467. 

(223) Duek, E. A. R.; Depaoli, M. A.; Mastragostino, M. Adv. Mater. 1992, 4, 287-291. 

(224) Pyrasch, M.; Tieke, B. Langmuir 2001, 17, 7706-7709. 



166 



(225) Xu, J. J.; Fang, H. Q.; Chen, H. Y. J. Electroanal. Chem. 1997, 426, 139-143. 

(226) Bharathi, S.; Yegnaraman, V.; Rao, G. P. Langmuir 1995, 11, 666-668. 

(227) Itaya, K.; Ataka, T.; Toshima, S. J. Am. Chem. Soc. 1982, 104, 4767-4772. 

(228) Ferlay, S.; Mallah, T ; Ouahes, R ; Veillet, P.; Verdaguer, M. Inorg. Chem. 1999, 
38, 229-234. 

(229) Champion, G.; Escax, V.; Moulin, C. C. D.; Bleuzen, A.; Villain, F. O.; Baudelet, 
F.; Dartyge, E.; Verdaguer, N. J. Am. Chem. Soc. 2001, 123, 12544-12546. 

(230) Escax, V; Bleuzen, A.; Moulin, C. C. D.; Villain, F.; Goujon, A.; Varret, F.; 
Verdaguer, M. J. Am. Chem. Soc. 2001, 123, 12536-12543. 

(231) Shimamoto, N.; Ohkoshi, S.; Sato, O.; Hashimoto, K. Inorg. Chem. 2002, 41, 678- 
684. 

(232) Guo, Y. Z ; Guadalupe, A. R; Resto, O.; Fonseca, L. F.; Weisz, S. Z. Chem. Mat. 
1999, 11, 135-140. 

(233) Lafiiente, C; Mingotaud, C; Delhaes, P. Chem. Phys. Lett. 1999, 302, 523-527. 

(234) Buschmann, W. E.; Paulson, S. C; Wynn, C. M.; Girtu, M. A.; Epstein, A. J.; 
White, H. S.; Miller, J. S. Chem. Mat. 1998, 10, 1386-1395. 

(235) Neff, V. D. J. Electrochem. Soc. 1978, 125, 886-887. 

(236) Lundgren, C. A.; Murray, R. W. Inorg. Chem. 1988, 27, 933-939. 

(237) Kulesza, P. J.; Doblhofer, K. J. Electroanal. Chem. 1989, 274, 95-105. 

(238) Kulesza, P. J.; Malik, M. A.; Miecznikowski, K.; Wolkiewicz, A.; Zamponi, S.; 
Berrettoni, M.; Marassi, R. J. Electrochem. Soc. 1996, 143, L10-L12. 

(239) Gao, Z. Q. J. Electroanal. Chem. 1994, 370, 95-102. 

(240) Herren, F ; Fischer, P.; Ludi, A; Halg, W. Inorg. Chem. 1980, 19, 956-959. 

(241) Abrahams, B. F.; Hoskins, B. F.; Lam, Y. H; Robson, R.; Separovic, F.; 
Woodberry, P. J. Solid State Chem. 2001, 156, 51-56. 

(242) Kim, J.; Whang, D.; Lee, J. I.; Kim, K. Chem. Commun. 1993, 1400-1402. 

(243) Kitazawa, T.; Kikuyama, T.; Ugajin, H; Takahashi, M.; Takeda, M. J. Coord. 
Chem. 1996, 37, 17-22. 

(244) Kitazawa, T.; Gomi, Y.; Takahashi, M.; Takeda, M.; Enomoto, M.; Miyazaki, A.; 
Enoki, T. J. Mater. Chem. 1996, 6, 119-121. 

(245) Ham, W. K.; Weakley, T. J. R.; Page, C. J. J. Solid State Chem. 1993, 707, 101- 
107. 

(246) Rayner, J. H; Powell, H. M. J. Chem. Soc. 1952, 3 19-328. 

(247) Woodward, J. D.; Backov, R.; Abboud, K. A; Talham, D. R. Acta Crystallogr. 
Sect. C. 2001, 57, 1027-1029. 

(248) Kou, H. Z.; Si, S. F.; Gao, S.; Liao, D. Z.; Jiang, Z. H; Yan, S. P.; Fan, Y. G; 
Wang, G. L. Eur. J. Inorg Chem. 2002, 699-702. 

(249) Constable, E. C; Lewis, J.; Liptrot, M. C; Raithby, P. R. Inorg. Chim. Acta 1990, 
178, 47-54. 

(250) Bowmaker, G A; Kennedy, B. J.; Reid, J. C. Inorg. Chem. 1998, 37, 3968-3974. 



BIOGRAPHICAL SKETCH 

Jeffrey Thomas Culp graduated in 1988 from Rocky Grove High School, 
Franklin, PA. After graduation, he enlisted in the Navy as an aviation electronics 
technician. Jeff served four years with tactical electronics warfare squadron VAQ 140 
stationed at NAS Whidbey Island, WA. His squadron deployed aboard the aircraft 
carrier USS Eisenhower where he worked as a flight deck avionics troubleshooter. His 
naval service took him many places including the Caribbean Sea, the Arctic Ocean, the 
Mediterranean Sea, the Suez Canal, the Red Sea, the Persian Gulf, and numerous 
countries along the way. His squadron was one of many called to duty during the Gulf 
War's Operation Desert Shield and Operation Desert Storm. Jeff was promoted to Petty 
Officer Second Class shortly before his honorable discharge in the summer of 1992. 

Jeff started his undergraduate studies at Clarion University of Pennsylvania in the 
spring of 1993 as a chemistry major. He was awarded the CRC Freshman Chemistry 
Achievement Award after his freshman year. After graduation with a bachelor's degree 
in 1997, Jeff entered graduate school at the University of Florida where he was awarded a 
Grinter Fellowship as an entering graduate student. He conducted research in thin film 
materials under the guidance of Professor Daniel R. Talham and received his Ph.D. in 
chemistry in the fall of 2002. He has presented his research at national meetings and has 
published many of his results in peer-reviewed journals. 



167 



I certify that I have read this study and that in my opinion it conforms to 
acceptable standards of scholarly presentation and is fully adequate, in scope and quality, 
as a dissertation for the degree of Doctor of Philosophy. 



Daniel R. Talham, Chan- 
Professor of Chemistry 

I certify that I have read this study and that in my opinion it conforms to 
acceptable standards of scholarly presentation and is fully adequate, in scope and quality, 
as a dissertation for the degree of Doctor of Philosophy. 




Michael J. Scott 

Associate Professor of Chemistry 

I certify that I have read this study and that in my opinion it conforms to 
acceptable standards of scholarly presentation and is fully adequate, in scope and quality, 
as a dissertation for the degree of Doctor of Philosophy. 




Philip J. Brucat 

Associate Professor of Chemistry 

I certify that I have read this study and that in my opinion it conforms to 
acceptable standards of scholarly presentation and is fully adequate, in scope and quality, 
as a dissertation for the degree of Doctor of Philosophy. 




KhahT A. Abbdud 
Scientist of Chemistry 

I certify that I have read this study and that in my opinion it conforms to 
acceptable standards of scholarly presentation and is fully adequate, in scope and quality, 
as a dissertation for the degree of Doctor of Philosophy. 




<0 



Mark W. Meisel 
Professor of Physics 




,c%? 



UNIVERSITY OF FLORIDA 



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